In the movie Predator (1987), right after Blain was killed, Dutch and his entire team fired at the creature. Specifically Mac said he fired two full packs straight at it. And we also know creature was hit and it bled.
So, the creature is strong enough to withstand such powerful gun attack.
How does such a creature get badly wounded so that it decided to commit suicide when a wooden log falls on it, that too a wooden log which Dutch was able to raise himself to that height!
First of all, the team fired at it. This means in the general direction of the Predator. Yes, they leveled the jungle in doing so, but between the Predator's agility and its sophisticated camouflage, it was able to evade most of the damage laid down by the team. This doesn't mean it wasn't hurt, as we saw there was blood on the leaves, which Anna later shows to Dutch.
Secondly, Newton's first law of motion states:
Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it.
The external force was the sudden stop as the log hits the Predator. This was a heavy log dropped from a substantial height. By the size of it, I would guesstimate it to be around 300-400lbs. As this accelerates moving towards the ground, it will build up tremendous amount of Kinetic energy. Since we know the creature can be injured, the ultimate force of the log would put a real hurting on the Predator. He's big. He's strong. He's not unstoppable. As far as Dutch getting the log into position, if you see, he has wedged himself into position using the rocks and such for stability. This and his manly-man strength were enough for him to lift the log to set the trap. The log falling was more than enough to crush the Predator.
Also, the Predator wasn't so much committing suicide. By blowing himself up, he was doing two things. First, this was one final attempt to take out his opponent. We know this because he mimicked Billy's laugh as though to say, "Haha, get away from this!". Secondly, it is doing it to completely obliterate any and all trace of itself.
Consider too, that it was already hurt, that when it was hit by the log, it was not expecting it, so it took the damage without mitigation, and within the lore of the screenplay, it likely triggered a protocol (possible capture/death) that required it to remove evidence of its existence.
Then there is this: screenwriters are God. :-)
