mort- is the root for "death, (be) dead".
So mortigi is "to kill" and mortigo would be "an act of killing".
But killing by itself is not murder, that's why murdo is not the same as mortigo.
For example, if your pet hamster crawled onto the train tracks and got run over, then the train mortigis the hamster but it didn't murdi it.
That would depend on who or what is being contract killed.
A soldier is, effectually, a "contract killer" for a country or ideology - but calling a soldier a "murderer" is generally not done. But someone who kills for hire (for the mob, or other extra-legal entity) is typically considered, for legal purposes, a murderer - even if, to that killer, it's not seen as being any different than pest removal. He might call himself mortigisto, but the law would still consider him murdisto.
Thanks for the questions which I have to think about a bit.
The basic meaning of "tro" given in PIV (vortaro.net) is
- Partikulo, signifanta: «pli ol sufiĉe; pli ol necese; pli ol ĝustamezure»
- A particle meaning "more than enough, more than necessary, more than the correct measure"
Adding "multe" would not clarify the original sentence and so would be superfluous.
There is a czech movie named Four murders are enough, darling https://eo.wikipedia.org/wiki/Kvar_murdoj_sufi%C4%89as,_karulo