Psychology / Psychiatry

Well, if you've seen that kind of mental shift before in a live-action performance, just borrow the same kinds of cues for it in a animated version.
If it works in one, it'll work in the other.

Just make the motivations clear--why is the shift happening? Is it a product of a set of values, or specific beliefs? Is it triggered by something internal or external? Is it a mental aberration, or some kind of mental sickness?
If you can answer those, then you can get the impetus of the course you need to take.

If someone is going to go from a homicidal frame of mind, and/or back....what takes them there in the first place? Is it revenge? A sense of duty? Or some kind of mental illness?
No-one "just kills" someone else......there's very strong underlying "locks" in most healthy people that prevent that sort of thing. Usually what brings on homicidal urges are a set of events and motivations that push a person to kill.
There's either the divesting of emotions and conscience regarding guilt prior to killing in cold blood, or the wrestling of emotions and conscience after the fact if its homicide in the heat of passion.
Of course, it also depends on the characters. Animals often kill without passion or consideration (aside from feeding, or basic dominance), as do machines ( because machines have no emotions or passion)--but people almost always consider the act either before or after--unless there is something wrong with them.

Keep in mind that the key to most character traits is like most animation itself--its all cause and effect. I find that if a character displays a cognitive choice prior to an action, or as a result of an action--usually the acting ( if its done well) sells the motivations or whatever the action is.

If a callous, cruel bastard nonchalantly pushes a fellow off a cliff to his death, he'll tend to have some prior "signpost" emotions that support the inevitable action. Its like laying the foundation of the act for the audience to see. You might do this by showing specific kinds of indifference to things, or having the character say specific lines. You can have them intonate or emote in specific ways to suggest that they care so little that they can build to the point where they will kill without remorse.

If they are going from killing someone to being happy, then you probably need to set things up so the act of killing becomes totally justifiable. Thus the shift from the killing action to the character being happy works ( say, they have rescued a loved one) because the audience can see it makes sense in terms of common mores.

But just going from killing someone to being happy ( or bursting into song) doesn't make logical sense, because the emotional switch is too abrupt.
If they kill, then break down in remorse, then are comforted and THEN brighten in happiness--then there's a logic train that follows and makes sense.
If the character is like Wolverine, who at one point was supposed to be the guy that casually disembowels someone at the breakfast table, and then wonders what the commotion is all about--that character will read as being sick (or at least unstable). This is because without any motivation for killing, there is no logic-train to justify the action--so the audience considers it to be a wrongful act or product of some kind of sickness.