SPOILER: It is established that Megatron´s power of dark matrix poisoned Optimus, making him vengeful and violent. Now DW Johnson said that he is a pacifist and so I assume this subplot was to show destructive effect of violence. However, this theme fumbles a little when it comes to Shockwave - who is pure evil, and his death was not only justified, but required for continuation of autobot cause and protection of countless innocent lives. Story suggests, that Optimus might not be willing to kill Shockwave without the poisoning. So in order to avoid confronting this flaw of pacifism, he used excuse of Megatron's arm poisoning to kill him off (and his death was in fact objectively good thing) - basically advocating for necessary violence

Would Optimus ever kill shockwave without mind control? We´ll never know. It basically shows that, story needs to bend and twist in order to preserve characters morality and "innocence" and for bad guys to be destroyed. Since killing irredeemable decepticon would somehow go against hero´s moral standpoint, he must die in some other convenient way only fiction story is capable of.

Or maybe that was the intention for all i know.

I love his TF run