My party just completed running the White Plume Mountain dungeon. It was the first real D&D campaign that we ran, after a short intro campaign to get the basics down. Our DM is experienced and played for quite a while. Everything went well until the end when apparently I made a decision that is said to be "evil." But, what I would have considered for the greater good.

This is mainly a question for those who have run the campaign and how you would handle it, or where to go with the characters from here.

So, those who have completed the dungeon know that a baby materializes once you place all the collected magical weapons of Keraptis. The whole dungeon has been spent telling us how evil and horrible Keraptis is and we must do what we need to destroy him. With that in mind, I saw the baby as inherently evil and so as a mage I cast Disentegrate and killed it. We then killed one of the other characters in the room and the last character disappeared. And, at that point it was kind of over. I had inadventantly derailed the ending.

I've been told that there will be consequences for my actions as well there will be consequences for my party members who did nothing to stop me, and who did not intervene and kill/subdue me afterwards. Everyone in the party agreed that there was no way to tell that they baby wouldn't be evil or that it should be saved.

I just want to get any other opinions on how you would have handled this as a DM or as the players, and if this really should be so out of character for a chaotic good character.

Tl/DR: I disentegrated baby Keraptis, thus changing the outcome of the ending, and it was said to be an evil act.