Can never understand them sparing.... (spoilers)
Walt. Uncle Jack and his crew had $80M free and clear and all they had to do was add Walt's body to the grave. Instead he gave him $10M and let him go to potentially tell the police and/or come for revenge for killing his brother-in-law and taking his money. Which is exactly what he did.
People talk about how well-written the show is and maintains the quality, but that just made no sense whatsoever. Honestly, Walt being killed then and then Todd killing Skylar at Lydia's behest would have made a more realistic and dark ending to the series over the Hollywood stuff.