Basically Eugene said he had to do that or else Dominic would have killed his old master b...

Basically Eugene said he had to do that or else Dominic would have killed his old master but apparently he didn’t know that it would cause Dominic to fall into a trap. So he decides to go back to his master and sleep with him to find and save Dominic (which is hella stupid). Then after he succeeds in making a bond with his master he thinks Dominic is gone so he chooses to kill his master and himself (which is even more stupid because ml got into that mess in the first place because mc didn’t want him to kill his master-)