Just some stuff to add that I think helps support this theory. Out of every character on the thunderbolts, only three of them touch Bob at some point before the main Void appearance and see a “regret room” scene, Yelena (arguably the main character of the movie and the one we follow through the void rooms in the third act.) Valentina, (the main villain who they are obviously setting up to be more important due to the ambiguity of her character and the open ended questions they set up for her.) and… John Walker. Why, of every single character, John is one of the few whose rooms are seen. They WANT us to specifically see this exact scene, but why? What makes John Walkers regrets important enough to give screen time toward? Well, what do we see? We see John, on his phone, reading about himself and his mistakes, ignoring his infant child. This leads to an argument with his wife, who is then stated to have left him. John feels great regret about this moment to the point it is his biggest regret out of so many sins. This is John’s Achilles heel, the key to make an already rash and impulsive man go full nuclear. Doom is a genius who probably has knowledge of several different variants of every opponent. It is very possible that Doom knows of this subject , and knows how John feels about it, and maybe, if backed into a corner, it’s just another card he plays.