It is evident that that which is necessitated need not be necessary; it can be contingent. For example, an object's being scarlet necessitates its being red even though its being red is a logically contingent state of affairs. A state of affairs S is logically contingent just in case it is possible that S obtain and possible that S not obtain. The possibility in question is broadly logical: there is no internal contradiction or incoherence in the concept of the STOA's obtaining/nonobtaining. 'No internal contradiction' means no contradiction when the STOA is considered by itself, leaving out of consideration things and conditions external to the STOA. ('STOA' is a cute abbreviation of 'state of affairs.' Some use 'SOFA' which is entirely too mundane for my taste: I like classical allusions.)
I believe that Leibniz would agree with what I have just said, though he couches the point in a terminology which is not the best possible. He speaks of absolute and hypothetical necessity. Thus he would say that the object's being red, though a logically contingent STOA, is hypothetically necessary as opposed to absolutely necessary. I don't like this way of talking for the simple reason that the hypothetically necessary is precisely not necessary, but contingent. (This makes 'hypothetical' in 'hypothetical necessity' an alienans adjective.) I prefer to speak of the necessitated rather than of the hypothetically necessary. But let's not quibble over terminology. Zu den Sachen selbst!
Suppose determinism is true and an action of mine is determined by previous and present casual conditions including reasons (motives). If follows that the action is necessitated by these conditions together with the laws of nature. But as I just got finished saying, the necessitated need not be necessary: my action, though determined and thus necessitated, is contingent. It is as clear as day that my action's being necessitated does not confer upon it (absolute) necessity. Or in Leibniz's jargon: an action's being hypothetically necessary does not confer upon it absolute necessity.
Now suppose my action is 'morally significant.' Crooking my right index finger is not a morally significant action; but it it would be if the crooking were the pulling of the trigger of a gun aimed at a bipedal varmint. The question is: Am I morally culpable if I shoot a man down in cold blood if determinism is true? Or does the truth of determinism relieve me of moral culpability? I say it does, Leibniz says it doesn't.
My thesis is that Leibniz's reason for saying that the truth of determinism does not relieve one of moral responsibility is a bad reason. His reason is that an action's being determined does not make it absolutely necessary. This is a bad reason since it amounts to an ignoratio elenchi. The issue is not whether the action is contingent or necessary: all will agree that it is contingent. The issue is whether an action that is determined and indeed predetermined by a set of conditions some of which antedate the birth of the agent can be said to be free in a sense that supports attribution to the agent of moral responsibility. This is a very tough question, and not one that can be resolved by pointing out what is obvious, namely, that an action's being necessitated does not make it (absolutely) necessary.
If you carefully study the following excerpt from an abstract of L's Theodicy, you will see, I hope, that I am right in my criticism. I have added bolding. The entire document is available here.
III. Objection.
If it is always impossible not to sin, it is always unjust to punish.
Now, it is always impossible not to sin; or, in other words, every sin is necessary. Therefore, it is always unjust to punish.
The minor of this is proved thus:
1. Prosyllogism.
All that is predetermined is necessary.
Every event is predetermined.
Therefore, every event (and consequently sin also) is necessary.
Again this second minor is proved thus:
2. Prosyllogism.
That which is future, that which is foreseen, that which is involved in the causes, is predetermined.
Every event is such.
Therefore, every event is predetermined.
Answer. I admit in a certain sense the conclusion of the second prosyllogism, which is the minor of the first; but I shall deny the major of the first prosyllogism, namely, that every thing predetermined is necessary; understanding by the necessity of sinning, for example, or by the impossibility of not sinning, or of not performing any action, the necessity with which we are here concerned, that is, that which is essential and absolute, and which destroys the morality of an action and the justice of punishments. For if anyone understood another necessity or impossibility, namely, a necessity which should be only moral, or which was only hypothetical (as will be explained shortly); it is clear that I should deny the major of the objection itself. I might content myself with this answer and demand the proof of the proposition denied; but I have again desired to explain my procedure in this work, in order to better elucidate the matter and to throw more light on the whole subject, by explaining the necessity which ought to be rejected and the determination which must take place. That necessity which is contrary to morality and which ought to be rejected, and which would render punishment unjust, is an insurmountable necessity which would make all opposition useless, even if we should wish with all our heart to avoid the necessary action, and should make all possible efforts to that end. Now, it is manifest that this is not applicable to voluntary actions, because we would not perform them if we did not choose to. Also their prevision and predetermination are not absolute, but presuppose the will: if it is certain that we shall perform them, it is not less certain that we shall choose to perform them. These voluntary actions and their consequences will not take place no matter what we do or whether we wish them or not; but, through that which we shall do and through that which we shall wish to do, which leads to them. And this is involved in prevision and in predetermination, and even constitutes their ground. And the necessity of such an event is called conditional or hypothetical, or the necessity of consequence, because it supposes the will, and the other requisites; whereas the necessity which destroys morality and renders punishment unjust and reward useless, exists in things which will be whatever we may do or whatever we may wish to do, and, in a word, is in that which is essential; and this is what is called an absolute necessity. Thus it is to no purpose, as regards what is absolutely necessary, to make prohibitions or commands, to propose penalties or prizes, to praise or to blame; it will be none the less. On the other hand, in voluntary actions and in that which depends upon them, precepts armed with power to punish and to recompense are very often of use and are included in the order of causes which make an action exist. And it is for this reason that not only cares and labors but also prayers are useful; God having had these prayers in view before he regulated things and having had that consideration for them which was proper. This is why the precept which says ora et labora (pray and work), holds altogether good; and not only those who (under the vain pretext of the necessity of events) pretend that the care which business demands may be neglected, but also those who reason against prayer, fall into what the ancients even then called the lazy sophism. Thus the predetermination of events by causes is just what contributes to morality instead of destroying it, and causes incline the will, without compelling it. This is why the determination in question is not a necessitation - it is certain (to him who knows all) that the effect will follow this inclination; but this effect does not follow by a necessary consequence, that is, one the contrary of which implies contradiction. It is also by an internal inclination such as this that the will is determined, without there being any necessity. Suppose that one has the greatest passion in the world (a great thirst, for example), you will admit to me that the soul can find some reason for resisting it, if it were only that of showing its power. Thus, although one may never be in a perfect indifference of equilibrium and there may be always a preponderance of inclination for the side taken, it, nevertheless, never renders the resolution taken absolutely necessary.