To every such "Why ?" there are a myriad answers.

When a mysterious murder is committed, everyone seeks the motive.

Unless circumstance unquestionably provides the key of the enigma, who can tell?
It may be revenge for the foulest of wrongs.

It may be that the assassin objected to the wart on the other man's nose--and there are men to whom a wart is a Pelion of rank offense, and who believe themselves heaven-appointed to cut it off.

It may be for worldly gain.