Which Sometimes I failed to return the books, because I'm lazy. veya Sometimes I failed to return the books, because I was lazy. sounds the most natural?

If you are making a factual statement "Sometimes I failed to return the books, because I'm lazy." is better
because "I'm lazy" is in the present tense(you were lazy then & now)
but "was lazy" is in the past tense(you were lazy then but maybe not now).

But if you are apologizing for not returning the books you should use "Sometimes I failed to return the books, because I was lazy." because then it would sound like you were lazy in the past but from now on you would return the books.