Solid fuels have a lot of advantages for military rockets, but they have a significantly lower specific impulse. It took quite a few years to develop really long range solid fuel rockets like the Polaris and Minuteman missiles. The V-2 was designed to carry a 1 ton warhead about 300 kilometers. Trying to go with solid fuels in the 1940s would've been pushing the technology pretty hard for the time.

von Braun was a member of the SS. The central question is whether he knew about and had a hand in the concentration camp used to build the V weapons. If he did, then he was a war criminal. He denied knowing about it and perhaps that's the truth. Lacking hard evidence to the contrary, all I can do is take his word for it at the moment.