A fundamentally existentialist philosophy for living in the world on the part of both protagonists, would be Existence Before Essence. Donnie never have dreamt of doing anything bad to cause harm to anyone, especially those of his family members. His ongoing meetings with the rabbit keep telling him evil plans to ruin someone's day or terrorize their feelings. At the end when Donnie stays in his bed and the jet engine falls in his room, he dies. If he still lived, then he would do the bad deeds around people when he was in the tangent universe. But he ended up dying, so now he wouldn't be able to terrorize the lives of others. Whereas, Robert lived his life by fighting in the war and dying naturally from the fire that engulfed the barn with him inside.
Robert's role of fate vs free will made him become a hero to some because he was a caring person and had an affinity for animals. He was connected to them in way where he would care for them, like he would would care for his sister, Rowena. To some, he was a traitor because he didn't listen to Captain Taffler for certain reasons that made him upset and turn into a rebel. While Donnie was just a regular teen who had psychological issues seeing an imaginary rabbit that told him what to do and what not to do. He never imagined to do any wrong deed, it was just his conscience taking over his actions and thought process.
Robert believes that he is living for life. It's time that he is waiting for because he knows that his day will eventually end, but he doesn't know when. Donnie on the other hand is being suspected of by his doctor since he tells her of all his psychological issues, as well as his parents. His father tells his mother one night that Donnie was probably made to cause danger to others and do some kind of damage that could be a sign of the apocalypse.
The possibilities and meanings of exploring tangent universes/ parallel worlds/...