That would not have been out of character. Megatron has helped Optimus quite a few times through the franchise, most notably G1 and during the Unicron chapter in TF Prime.
People who say that Megatron's death was uneccessary never watch the movie closely. You can see Megatron raise his shotgun, he even fired at Optimus before that axe got slammed into his skull.
Love that piece when Megatron charges into battle... Absolutely epic!
@SamWinchester81 Rumors. Other than Optimus's confirmation, there has been no final news on any new or old bots. However, TF4 is now called a reboot since Wahlberg blurted it all out on the Leno show. So that may mean another return for Megatron, Starscream, Soundwave, Shockwave, etc.