pet rat died

[Lrptls]

Back in october when i use to work at another pet store, a lady came in with 2 rats she no longer wanted so i took them. they were both very sweet, especialy the rex rat. he is still very loving- he comes when i call him, loves being held and loves when i scratch him really hard. the other rat liked me but was never very happy about being held, he was fine being on my sholder and sometimes liked being pet. the rex was the older, bigger, dominent rat. they got in alot of fights, always about food. i would hear them running around and squealing, i'd run over and see the rex pinning down the other rat. when i gave them treats they always had to have one each which kept them from fighting.

but last night when i got home the regular fancy rat was dead and half of his head was eaten, he wasn't very old at all while the rex is very old and i know he will be leaving me soon. they had food and water and they lived in a 20 gallon long aquarium so i'm pretty sure they had plenty of space. i just can't figure out why he would kill his friend rat. they were almost always together, sleeping and cuddling so i'm not sure what happened for him to do that. if any one has any idea why rats would kill each other in this situation please tell me so i can make sure this never happens again.