Mario = The name Mario
Killer = Of course, Killer
64 = The number 64.
Now, put Mario and the numbers in "Mariokiller64". You would see Mario64 referring to Super Nintendo Mario64. As in, Mariokiller64 means killing Mario64, Now you ask "Why didn't you just use Mario64Killer." I didn't choose that because it sounded lame =D. Mariokiller64 generally means killing Super Nintendo Mario 64.
Mariokiller64 just made Mario jump off the castle to kill himself.