When it comes to the villains, Doctor Who episodes typically go one of three ways. They can be blown up or disintegrated (or defeated in some other way), they can escape with their lives, or they can sometimes - very rarely - turn good.
While the majority of Who villains never abandon their evil schemes and will normally push through to the bitter end, there are a few cases in the show's history where the bad guys became the good guys, whether the Doctor talked them into having a change of heart, whether it was self-motivated, or whether they sucked a human inside their casing and subsequently grew a conscience. Looking at you, Dalek Sec.
Even some of the Doctor's biggest and baddest alien foes are capable of changing for the better - as are the many, many human enemies that the various TARDIS crews have bumped into on their travels - proving that the Doctor is right to always look for the goodness in people.
There are no guarantees that these villains will stay all kind and sweet if they return in the future, but for now, let's focus on the times when their heroism shone through.