Yes it's possible to completely kill a phone by rooting/unlocking it, especially one that requires using an exploit to accomplish the unlock. It's called hard bricking a phone (as opposed to soft bricking, which is recoverable)
Unlike a PC, where the bios lives on a chip and the OS lives on a hard drive, with a smart phone everything lives on the same chip, just at different addresses. If you manage to overwrite the wrong address, you could scramble the phones bios, rendering it non repairable (without replacing the main board).
This was a bigger risk in the early days of Android, now days most manufacturers (notable exception is motorola) have made thier phones more hacker friendly, so it's a simple matter of connecting the phone to a pc and running a few safe commands.