Cannonball said it better than I could have.
But still gotta echo, I don't agree with the idea that the costume is an indicator of anything, other than the perp planned a way not to get caught at CCoC. But whether they expected to be concealing a murder, or instead didn't want to be caught for b&e, burglary, trespassing with an identity that was somehow known, or any number of other things that aren't murder, there's no way to know until we know who did it and why.
But still gotta echo, I don't agree with the idea that the costume is an indicator of anything, other than the perp planned a way not to get caught at CCoC. But whether they expected to be concealing a murder, or instead didn't want to be caught for b&e, burglary, trespassing with an identity that was somehow known, or any number of other things that aren't murder, there's no way to know until we know who did it and why.