It's a tough one this Hitler topic, and one we've explored before. I guess it's hard to be prescriptive about what might have happened to him, in that how you see it is going to depend on your beliefs as to the nature of the afterlife process. Not that our beliefs may necessarily be that close to the reality either...
If you take the ACIM type view a loving God did not create a reality in which events like WW2 and all the other horrors seem to happen - we (as in the collective ego of the beings separated from him) projected it as a means of suppressing our awareness of separation, and out of a mistaken fear of punishment for it.
As a result these events have no absolute reality, and any wrongdoing (non loving behaviour) is forgiven even before it occurs. If we could accept this then we would no longer feel the urge to project this reality, so rebirth into it would not occur - we would simply return to God/Spirit.
The problems develop in that most of us (and especially those inclined towards egotistical and non loving behaviours) cannot accept this at the deeper levels of mind, or even (usually) at the conventional intellectual level. The result is that we initially (as a result of our initial enjoyment of the freedom of the afterlife) create very positive experiences, but sooner or later the guilt kicks in (basically in the form of the life review - which may be something we ourselves create as a result of the separation of our awareness/essential self which then experiences the contents of our memories/personality/ego) and we start to experience negative stuff.
The resulting fear and possibly horror we generate at our past actions (our essential self truly 'sees') propels us into rebirth, with the resulting very high levels of horror/fear in the case of serious wrong making likely our rushing into highly undesirable life circumstances - and so the cycle continues as these circumstances drive us into further wrongdoing.
As talked above I guess mitigating factors like being a part of some sort of learning process motivated by spirit to assist our escape and arranged prior to birth (where spirit uses the circumstances of the reality we have created for the spiritual growth of many - but did not ab initio create the events), or unknowing error committed out of a lack of wisdom with genuinely no intention of harm, or a balancing of positive and negative outcomes, physical or mental incapacity/disability, or simply being a catalyst that triggered an already latent tendency in a population, or being a more or less realised person may well have an effect - maybe reducing the depth of our horror at review, and our fear of retribution - and hence may make possible rapid learning without rebirth into undesirable circumstances.
Intention as Kathy says is probably an important determinant of the situation.
Against that very fearful beings with no trust in the existence of any reality other than the physical may refuse to move on out of this world - and hang about for as long as they can as ghostly entities - possibly even trying to live off those still in physical life. Such beings may also have in life overridden their spiritual sight (such as it was) with a deliberate intention to do wrong - masking it, and making their fear even more intense.
Buddhism uses different language, but teaches something pretty similar about the bardos and rebirth - conceptualising it seems the fear and horror that we experience (at deeper levels of mind than we are normally conscious of) as the karma that not just drives us into rebirth and determines our birth circumstances, but also follows us into and on an ongoing basis creates our life. (logical enough if you accept that mind creates all)
Both systems suggest that karma/fear/guilt at separation are self rather than divinely inflicted, are capable of gradually being dissolved/lessened through appropriate spiritual practices like meditation (which by calming and creating space in the mind makes space for spiritual intuition/assistance from Spirit to get past the fear and resulting mental babble into awareness) and contemplation of teaching (which inculcates a correct 'view'), and are only truly transcended with enlightenment. (or the end of all belief in relative/dualistic existence, and true spiritual sight)
It's possible of course to see it all very differently. We can posit a God created Universe, and see it as a learning environment designed to create experience or some such similar output of value.
Or we can posit the existence of a God that punishes sin in proportion to its severity, and rewards good with a ticket to heaven.
It's surely hard to see Hitler as deserving of anything but love, compassion and understanding. He was born into unique circumstances which conspired with his personality and his belief in the use of power and force to bring the worst into existence, but it's not like it's very likely that it made him happy in life, and it's hard to imagine how he entered the afterlife without considerable issues to resolve unless he somehow received a lot of assistance or was a lot more realised than we're inclined to credit.
Please don't take offence at this, but even presuming that he was the proverbial monster we should perhaps be careful of desiring his damnation. First off the possibility is that we're simply attacking a projected and suppressed aspect of our own mind (there is only one mind), secondly we're training ourselves in an unhealthy and unloving attitude of mind, and thirdly we're simply reinforcing the possible belief of such a being (again actually an aspect of our own mind) that that love is not a viable basis for living.
Better that we forgive, and seek to envisage the positive/love informed outcome we'd like to see to his situation...