Another great episode.
More serious than last weeks "the things we do for love" (and slightly darker, straddling the gap between last weeks lighter episode and the prospect of next weeks offering.)
I liked the way the motive for the murder cycled through all the negative aspects: killed for his research, killed to change a grade, killed to cover up a secret...then at the end? It turned out that the wrong thing was done for the right reason. Love motivated the murderer. That entire interrogation room scene with the wife

and Beckett's reaction throughout
I also found it interesting that the wife had "killed" her husband with the prospect of preserving their forever. A negative action now...with hope for a positive action in the future. Rather like Beckett, who lies to Castle now...in the hope that they'll have their future together, that they'll have their forever ("always")
The scene where Beckett found Cynthia Hamilton dead, clutching the photo of herself & her husband? Had to bring back memories of her mothers death (and how her father nearly followed her mother...drinking himself into oblivion)
Last week "Heroes & Villains" brought us a lighter episode and a lighter view of love. This week it was the dark side of love mixed with loving longest even when all hope seems to be gone.
(I'll be back when I've watched it again...rather than at half four in the morning!)