#3 He has no other choice
Apparently, what Seth Rollins is doing isn't all wrong because he doesn't have any other choice except hitting other Superstars with steel chairs or using low-blows. The circumstances are against Rollins, who is surrounded with multiple enemies.
Baron Corbin is already a heel Superstar, and since he can choose a special guest referee for his upcoming match, Rollins could easily lose his title. And he also knows that once Lesnar is on his way, it wouldn't be easy to stop him, which is why he might have used a low-blow.
This way Rollins could prevent others from taking advantage of him. At Stomping Grounds, Corbin is going to face him for the Universal Title, and since he hasn't revealed who is the special guest referee, Rollins has to do things carefully. Otherwise, Corbin (or Lesnar) could easily beat him for the title, which is not what he would want to happen.