He knew what he was doing - se hacía? Why se?
Sabía lo que se hacía.
Como alguien que le ha visto en acción,
Hay que decir que Glenn sabía lo que se hacía
En esa maldita ciudad.
I gotta say that Glenn knew what he was doing (transliterated hay que)
2 Answers
Good question, Jeezz!
In this case, the sentence is exactly the same as:
...sabía lo que hacía
which is the normal (standard) way of saying it.
That "se" is an extra, and it sounds between informal, colloquial and emphatic. It doesn't fit well in all the contexts, and it's mainly spoken.
Standard: Tranquilo, sé lo que hago
Colloquial: Tranquilo, sé lo que me hago
Calm down, I do know what I'm doing.
Sabía lo que se hacía.
He knew what was being done.
Sabía lo que hacía ...... He knew what he was doing.