He's not dead. I bet the remains of my anal virginity (not much, but it's all I have) that he uploaded his conscience into Jack's helmet. Two reasons for this: 1) the bond between BT and Jack was a large part of the Titanfall 2 campaign experience, and it was really well done--what Halo 4 wishes it could be. Respawn knows they did good in this area and wouldn't shaft themselves by killing BT permanently. 2) at the end of the credits there's a short scene where the lights on Jack's helmet flash on and off briefly. I guarantee Respawn is hinting to us that BT is somehow not dead and has uploaded his A.I. into the helmet. And fun fact: BT stands for Buddy Titan