Only thing I've seen is that he was dating a trans M-T-F person, and that person was very cooperative with police. Although it makes you wonder about his gay comment engraved on the bullet.
No, it's quite common and less confusing than "Trans Woman" because some people not well versed in these topics will be confused by only describing the final gender.
I dunno, I said it's used a lot by bigots, then a bigot shows up and agrees with me (now flagged). If that isn't a sign a term isn't great, I don't know what is?
Judging yourself by how unscrupulous people react to you is a common pitfall - it poisons your perception of others and blinds you to yourself.
Tangentially: I was confused by the usage of the ambiguous word, "weird", as shorthand for "bigoted". I thought you were literally asking what "mtf" meant.
It's certainly not conclusive.