Keep in mind that an attacker that can change source code can add a backdoor where there is none. Backdoors are dangerous because their keys can leak. In the case of Dual_EC a leak would have been particularly dangerous because the attack enabled by having the backdoor's key is passive, so not easily detected.

A backdoor rekey attack, if it lets the attacker gain a foothold that allows them continued access after the attack is detected. That is really bad!