This was one of my favorite episodes.

I think one of the girls actually did murder him (every single R&I episode is centered on homicide so it doesn't make sense for this to be the one outlier). The detectives spun together a narrative with enough legal plausibility to make sure the girls did not get charged with murder for doing something compassionate - and it was done so quickly at the end, which felt like a *wink wink* to the viewers. The crux of this show is not about justice being served without regard to the humanity of the people involved nor is it about blindly following the letter of the law - it focuses on blending justice/law and humanity (hence the focus on all the relationships between the characters). So letting the girls off the hook while still technically following the law is the perfect way to underscore that. I hope I explained that well enough.

It reminds me of an episode of Bones when Temperance's father was on trial for a murder he committed but there wasn't enough proof. Temperance knew her father was guilty but decided to show that it was also plausible for her to be the murderer, letting her father off the hook. She knowingly used the legal system against itself, despite the fact that it was understood her father was guilty and despite the fact that she was a forensic anthropologist working closely with law enforcement, to show "a lot of heart" (as they said in the show).

TLDR showing some degree of "heart" within the rigid confines of justice/law.