Wed Sotah 9 (really end of 8)

We say that someone who is Chayiv Misah nowadays, that Hashem brings him a death that's similar to what he was Chayiv.

Tosfos asks: we know if someone kills willingly and another unwillingly, neither with the proper proof to Michayav them. Hashem brings them together, the unwillingly one mistakenly falls from a roof onto the murderer and kills him. Therefore the murderer dies and the other one now has witnesses to send him to Galus.

The question is: the punishment for murder is beheading, so why does he die by the person falling on him, similar to Skilla (stoning?)

Tosfos answers: it's still a fitting punishment, since usually someone kills by falling something on the person, so that's the way he also got killed. If the murderer kills by sword, then his punishment would be different, as appropriate for his deed.