The suitors put his wife Penelope in a terrible condition.  He was defending her honor by killing them.  She had to fend off their advances and protect herself and her child.  She acted honorably, but it was hard for her because her husband was gone so long and they couldn’t understand why she didn’t remarry.

From the perspective of the time in which he was living, Odysseus clearly had a good reason to kill the suitors.  They were, in essence, showing a lack of respect to him, to his wife, and to his son.  They had behaved badly toward his wife and son and therefore in that time and place deserved to be killed.