Starck is a nice character, but he does not fit the enemy image needed in a shounen show. He's more like a suicidial depressive guy, all of his story tells about deep depression.

Shunsui was the right guy to kill such an enemy, because he only does what he has to when needed.

Also when referring to Captain Strength, all captains are not equal in power/strength, explaining why Shunsui can be so strong. After all he's been around for a long time with his friend and Yamamoto. To live long in Soul Society, you need to be strong.