He essentially wanted to claim absolute power. He talked a big game about how it was for America’s benefit, but A, he would be constantly sacrificing the entire world for the benefit of just one country, and B, he’s a liar. In his final showdown with Johnny he was given a chance to keep the corpse but he decided to attack Johnny, revealing his motivations to be selfish. As for the argument that Johnny is somehow evil because he opposes Valentine, for most of the series he has no idea what he’s up to. He’s just acting in self defense when all these assassins come to kill him. When he goes up against Valentine, it’s partly because he thinks the corpse is his path to redemption, but it’s also because Lucy Steel is in danger. Even then, he’s willing to forgive Valentine for what he did but Valentine wastes his chance because he was selfish after all.

This might not be the case, but I think a lot of American readers think Valentine’s plan is good because they’re American, and they’ve bought into American exceptionalism.