Can Orcs think for themselves?
 

I thought that if Orcs weren't working for a powerful master they were pretty much mindless. But in LoTR, the Orcs that captured Merry and Pippin included Orcs from Moria who were working on their own, not for Sauron or Saruman.

Also, after Sauron was defeated after the Last Alliance, Orcs still organized and attacked Isildur. How is this possible?

Yes thay could think for themselves. How could you not be able to think and fight. All thay would know would be kill and kill everything in sight including other orcs. Thay could think but probaly only the basics like eat, sleep, and ally.

Orcs were originally corrupted elves. They inherited the traits of the elves, who were the first sentient creatures native to Arda. The orcs could definetly think for themselves.

that's so sad.

Orcs can definitely think for themselves. They can make their own decisions but normally are within the boundaries of their orders.(if they have a master) They were after all originally elves.