Minecraft check who killed the player?

I'm currently building a minecraft killer game and do not know how I can use commandblocks to check who killed a particular player.

Becomes difficult with cmd blocks, convert to spigot and then prefer to use a plugin. You will also need a lobby which, as we know it, can no longer be implemented without plugins.

Do you know a good plugin for it?

There are plenty of them. Just google or take a look at YouTube.