Finding out his hometown was destroyed was quite a blow to Drax. After escaping slavery the town was his first breath of fresh air. It allowed him to find himself again, not as a dangerous assassin. The crazy old scientist had taken Drax under his wing and was helping reintegrate him into society.
Until his death Drax regretted hunting down the Splurgoth ambassadors in Canada. He always felt he could have done something if he was there during the attack. From the time of the incident people tried to explain to Drax there was nothing even he could have done, but he does not deal well with things close to him being taken away. There are very few things that remind him of a time before he was a killer and in one fell swoop many of his memories were destroyed.
It was clear the crazy old scientist wanted Drax to work with his old classmates. He knew he would have to protect them as they sought vengeance for the destruction of their hometown.