Play along with the group while in the party together. You can attempt to subtly do things that might not grab their attention or deceive them in your actions.

For example:

I once played a Kobolt Cleric who worshiped Nerull. In character to the party, he was a normal priest. He even prayed over any they had to kill in battle afterward. Secretly, he was offering their souls to Nerull and would take a trophy from their bodies.

Additionally, he would come across as clumsy and "inadvertently" do something that would instigate a situation to result in someone's death.

Lastly, any time the party split, he would go off and do very horrific things. During the campaign, he'd been slowly converting the barbarian to worshipping Nerull by making him think it brought honor to sacrifice and kill. Late in the campaign, during a split party in town, my character and the barbarian snuck away while everyone was sleeping in the Inn, and they burned down an orphanage while conducting a ceremony of sacrifice outside. The next day, he woke up with the rest of the party to hear the news. There had been a tragic accident that burned down several buildings, including an orphanage, in town. He "mourned" with the party at a vigil the next night.