As far as I can tell (as reported on the TV, which isn't exactly clear on the details), Patrick stands accused of two incidents. In one, he was acting as a defense attorney, and managed to get a life sentence of a convicted killer reduced to something less than life. (The actual numbers aren't revealed in the ad.) In the other, he thought that a kid was innocent of a rape charge, and paid to have a DNA test done; the DNA test instead proved that the kid was guilty. Both of these, according to Healey, indicate that Patrick is "soft on crime".
Do they? I mean, what they *actually* say is quite a bit more nuanced. Healey's message here is essentially, "Yes, yes -- innocent until proven guilty and all that. But we wouldn't want someone who actually *believes* in that in office, right? That's not *strong*, after all. The purpose of government isn't to defend people, it's to prosecute evil-doers. So anyone who believes in defending people is clearly a weakling, and not suitable for public office."
It's pretty clear to me that the message of all of this is that Patrick understands how the justice system is supposed to work, and Healey doesn't. If you're a defense attorney, you do your damned job to the best of your ability -- the system hinges upon that. If you think someone's innocent, you try to prove they're innocent. Precisely how is either of these a bad thing?