The ultimate relationship killers, I believe, would be constant lies or constant cheating. One can forgive a partner for cheating once, but if it happens again and again, you have to ask yourself whether this person truly respects and loves you?
Furthermore, even in the absence of cheating, any lies could easily kill a relationship very quickly. Once you realize someone is lying to you, you start to question every single thing in that relationship.
Jealousy is literally a monster, and it will feed on your relationship and your chance to be happy until you are completely miserable.
For more mistakes, you can make to ruin your relationship, click on this link to an article that will assist you.