Wilhelm left him to die. We still don’t know exact details, but since Wilhelm had Dietrich’s fabric piece (from Rein) it means that he either stole it from him before the battle knowing that he would probably die, or found him injured/unconscious and took it before leaving him for dead. Wilhelm believed that Dietrich was dead, but just MIA. Rein didn’t assume anything. Wilhelm’s story doesn’t make sense compared to the accounts of Dietrich, his now wife, and others who were in that military campaigns, so she was able to surmise that the timing of him regaining his memory, the fact that he knew the future before the ambush that supposedly killed Dietrich was, and that he had to have taken the fabric from him while he was alive (it was attached to Dietrich’s sword, but only the cloth came back). They all have trust issues, but she really tried to trust him and show support and affection, but when she had suspicions after discovering the cloth, he denied everything, and despite the red flags, she chose to trust him again as long as he came clean. Then she found out he lied, and that he was able to predict and mitigate every attack/battle except the ambush that killed Dietrich. He blew his shot when he arranged the death of her closest friend, a man who tried to support and teach him.
I don't even know who to trust anymore lmao. Does Wilhelm don't really know that dietrich will die? Or is it intentional? Either way fl would also jump into conclusions immediately, she's quite upset that ml doesn't have any trust for her when she's also the same for the ml