不错了.起码除了were就没别的错了啊.再稍微改一下：
A student was stabbed to death by his roommates, all because he delayed in answering the door (all because he did not answer the door promptly).
all becase有“就因为”的意思
用whom..可能更紧凑一些：
A student was stabbed to death by his roommates whom he did not promptly open the door for.