I checked the RD again. It is hard to tell if the hanger is bent by eyes. And the inner plate of the cage is deformed at upper pulley side. I figured maybe the deformation at the pulley's thread caused the misalignment of the upper pulley. I tried to deform it back a little. But this does not fix the problem.

I found there are scratches on the derailleur body. So the bike definitely hit the ground before. I bought this bike off Craigslist. The seller told me he replaced the hanger. But I did not have a clue what is that when I bought it.

Now, I can shift gear very smoothly in the first 7 cogs. However, RD does not engage into the 8th gear (smallest) no matter how good I adjust the derailleur. It looks like the upper pulley is bent inwards when the cage tried to move outwards.
I guess maybe I should let the shop take a look.