Was Bilbo right to spare him? Would Gollum have spared Bilbo if he could have caught him? Did Bilbo have any right to keep the ring?

I don't think Gollum would have spared Bilbo, Gollum was too far gone under the power of the ring. I think Bilbo had a quest, and in that sense he had a right to keep it. THe ring, however, can't be possessed without severe consequences to one's spirit, mind and soul. Bilbo needs to destroy it.

I think Bilbo was right to spare Gollum in that it was the empathetic thing to do. Bilbo knows what power this ring possesses and he could have very well ended up like Gollum if he had not been warned. That being said Bilbo should know to never trust Gollum.