Why was the Behemoth still alive if I defeated it?

After a couple of tries I "defeated" the Behemoth, or so I thought.

I got the David's Challenge message, 5 Skill Points and some XP...I didn't get the reward I wanted which was the only reason I was fighting the Behemoth.

After all of those notifications I fly over to the Behemoth and it moved so I jumped back in the helicopter (I defeated it with a Rocket Chopper) and start trying to fly away...and it hits me with a SAM.

I survived the crash...however I literally landed next to a patrol who started shooting me and killed me.

Could someone explain what happened?

Like I said, I didn't get the one thing I wanted so I don't know if it's a bug and I was supposed to get it or are there levels to "defeat" I wasn't aware of?