Only if it is proved to a criminal standard (i.e. beyond reasonable doubt) to be intended to encourage an act of violence. It it is an expression of anger it is legitimate free speech.

Your comment has deliberately omitted the context needed for honest discourse. Thus, one can only conclude you are trolling.

So did the one they're replying to. They're just replying like-for-like.

It's typical that when someone is arrested for "X action with Y detail" (e.g. buying a knife with intent to kill someone) people who oppose the arrest will only state the X (and for some reason this works). To correct the record when someone says "I don't expect to be arrested for buying a knife", "I do expect to be arrested for planning to a murder" is a correct response.