We aren't told that much about Bob, except that he is a leader of the Socs and is clearly rich and handsome. He is also the gangleader who beat up Johnny so badly before the novel began, leaving him haunted by his experience. He wears rings to cause more damage when he is fighting.
It is in Chapter Four when Bob is killed by Johnny. However, any attentive reader will be able to identify a whole series of factors that caused his death. To merely focus on Johnny as the cause is to ignore many other different issues. Firstly, the chapter makes it clear that it was the Socs that picked this fight. Also, Johnny only stabbed Bob to help Ponyboy who was being dunked in the water. In addition, Johnny would not have been so nervous and on edge had he not been beaten up by Bob and his gang in the first place. Equally, if Bob hadn't been drunk, he might not have been so pungnacious. So, even on face level, we can see that there are a number of different factors to consider.
However, alongside these issues, we have wider and deeper societal issues. If there was not such a divide between the rich and the poor, such conflicts would not occur. If parents were more involved in their children's lives, perhaps they would not let them go out and get into these dangerous situations. The novel presents us with many different factors that resulted in Bob's death, leaving it impossible to point the finger at any one individual or group.