They can be justified in some capacities.Mickey had no business getting away with killing Maureen, really. Especially considering what happened with James Holmes, who perpetrated a far worse and more bombastic crime in what was essentially the same setting, and was apprehended very quickly.
Strange to contemplate how the motives of the various killers wound up being highly personal ones for the most part (I have an argument that Mickey's attorneys might attempt to paint him, if he was apprehended, as a mentally ill pawn of the vengeful Mrs Loomis - what success this might have is open to debate of course).
Which begs the additional question - just what was it made those after Billy and Stu invariably ALSO opt for the Ghostface costume as a disguise? The more superstitious might speculate that the very character/icon itself is cursed in some way and worsens the existing grudges and murderous impulses of those who don the mask.
Roman's is the most obvious, Scream 3 revealed he kinda egged Billy and Stu into their murderous rampage (Hell, it could have been his idea).
The others all seemed to base their plans around Sidney, in essence, they were making a sequel.