He doesn't always take someone with him, against a similar powered player, the shanigami only deals about 50-80% of the target's max hp (depend on who it is). Granted, there usually is someone who is damaged that you can take with, and if you do, it can't even be revived, but for technical correctness, it must be said that it's NOT a instant kill.

Also, the skill CAN be interrupted, thou it is prompt so not very easy to do so.