Considering he’s a member of a clone race, Commander Strax is a remarkable individual. We’ve seen the Doctor encounter many Sontarans and whilst every other member of this belligerent species has been guided by a single-minded passion for war, Strax is a nurse. And a good one.
We’ve witnessed him save the life of a child in a warzone, give medical advice to a weary soldier and Strax even had himself gene-spliced so he could be fit for all nursing duties. When baby Melody appeared to need a feed he proudly declared, ‘I can produce magnificent quantities of lactic fluid!’
He told a puzzled Captain Harcourt why he was helping those he would normally attempt to destroy. "I serve a penance to restore the honour of my clone batch," he explained. "It is the greatest punishment a Sontaran can endure… to help the weak and the sick." It’s strongly implied that the Doctor invented this ‘punishment’ and was responsible for Strax’s switch from military commander to medic.
But the warrior within Strax still remains and he is no pacifist. At the Battle of Demons Run he threw himself into the fight with a gutsy enthusiasm and he helped the Doctor defeat the Great Intelligence in 1892.