hmmm, one of my fave is the myth about Poseidon's two sons who are both cyclops or giants. They are the Aloadea twins (namely Otus and Ephialtes)
They just loved each other so much and cared for each other. They are troublesome that one of their "great" deeds is to imprison Ares on a chest. Hermes came to save Ares and Hermes succeed. Not yet contented, they planned to pile Mt. Ossa to Mt. Pelion (don't know if I got the names correctly, sorry) or the other way around. Zeus got tired of this twins' doings and decided to kill them by using his thunderbolts, but the twins' father Poseidon intervened and told Zeus that he'll talk to his sons. For some time, the twins seem to listen to their father's plea, but they didn't.
They decided to carry off the goddess they love, one of them decided to kidnapHera, the other one Artemis. I don't know what happened but they decided to have Artemis first. They saw Artemis and followed her through the woods, but the goddess knew what they're thinking. Finally they lost sight of her and instead found a lovely doe(not sure what kind of animal) and decided to capture it, both forgot that they are there to have Artemis. The two separate ways to capture the animal faster. Both cast and throw their spear towards the animal but they were both shocked to see that their spears stuck, not the animal, but instead they stuck each other, killing them.
So in the end, it seems that they kill each other lol
Sorry for this long post.