Why did he say "Ka'asher Diber"?
Radak: Kill him - there is no claim not to. He killed two head generals b'Mezid; "take him from My Mizbe'ach to die" (Shemos 21:14)!
Malbim #1: Let the Sanhedrin kill him (for killing Avner and Amasa), and he will be buried in his ancestors' gravesite. Initially, Shlomo wanted to kill him for rebellion (supporting Adoniyahu).
Malbim #2: Shlomo accepted the curse (refer to 2:30:1:1).
Why did he say "u'Kevarto"?
Refer to 2:28:3:5.
Radak: Bury him honorably - he was a great man and a head general. Shlomo did not need to command so about Adoniyahu, who was the king's son. Surely he will be buried honorably!
What is the meaning of "va'Hasirosa Demei Chinam?"?
Malbim: The Sanhedrin will judge him for killing Avner and Amasa.
