Bluebirds Vs Tree Swallows!

In my whole life of studying birds and bluebirds I have never seen something as ironic as this story! I went to work on Monday morning after my first brood of Bluebirds had fledged their nestbox and was checking both boxes to see if there were any signs of new nesting materials in the box! I checked the first one and there was some nesting material, I checked the second one and I found a tree swallow in the box! I decided to grab its tail thinking it would react in some manner, but I found it to be limp and dead, strange! After realizing the bird was dead I observed it's body and I found lacerations all over its head! I'm assuming that the Tree Swallow was in the wrong place at the wrong time and the male Eastern Bluebird didnt want the male Tree Swallow in his territory! I just have never seen such a behavior! So as I blog this if anyone cares to comment on this issue, I'll be interested to hear some input!