Theodoret of Cyrus

Some people blame Solomon because he killed his brother. Now the ways of life of people are different. Some of them certainly practice the highest form of philosophy; others pursue that virtue which is called political or civil; others manage the kingdom or hold power militarily. It is necessary to judge each of them according to the way of life that they follow. Therefore an apostolic or prophetic perfection cannot be expected from Solomon, but only those actions that are appropriate to kings. He knew that Adonijah desired to gain supreme command. In fact, he had tried to take hold of the kingdom. When he attacked him the first time openly, he still forgave him and promised him that he would be safe if he behaved with modesty. But after he asked for the partner of his father, he did not grant this to him because Adonijah was opening the way to tyranny. Therefore Solomon ordered him to be killed, as he was concerned for the tranquility of his kingdom.