actually he can, but you really have to follow me here. Its not about the character of the wielder its how they are seen in the eyes of oden who put the enchantment on it. Oden was once like thanos in the sense of he took over the realms and then went on to be peaceful, like Thanos did when he used the stones and then went to that planet and lived peacefully. So if Oden can wield the hammer despite once being a tyrant then Thanos could to. Another requirement for Mjolnir is to be willing to sacrifice yourself and that which you love for the greater good. Thanos did this when he sacrificed gamora and sacrificed his life by using the stone not ONCE, but twice and he even admits it almost killed him. Now you may be thinking, dont you have to be willing to protect the realms and do whats best for them? Yes you do and Thanos does this, sure his methods are frowned upon but in his eyes he is helping the universe. Think about it his methods of killing with the snap arent violent or painful so hes merciful and he believes that in wiping out half the universe he helped reset humanity and gave them a second chance. Hes 100% worthy you just need to know your stuff.
Tony and Natasha are worthy too so let me know if you want that explained :)