It doesn't MATTER. He dies, they lose the buff, they die. End of story. Or I guess to put it a different way, he dies first because he only has 2 health, they are left as 3/1, but as soon as he leaves the field, which is immediately, they lose the 1/1 and die because they have 0 health.
The same thing happens with Pack leader. If WP creatures are left with three of less health, and aren't otherwise healed or buffed, you don't even have to KILL the Pack Leader, just remove it from a combat zone, and they lose the 3/3 and die.