Several factors come into play.
Firstly, X-Men 3 does not tend to be considered canon:
According to Lauren Schuler Donner, one of the producers of all of those films, you can now safely consider X-Men 3: The Last Stand and X-Men Origins: Wolverine as non-canon. She explains this at the premier to X-Men: Days of Future Past:
“Just forget about ‘X3’ and the first ‘Wolverine’ - forget about that, too!“
From this question: https://scifi.stackexchange.com/a/106178/3804
Secondly, the Beast we see in ''Days of Future Past'' had seen the idea of a cure go wrong too often. Firstly with himself, then Mystique's reaction to the mere idea. Finally Bolivar Trask using the same cells, that helped him create his first serum, for the Sentinels. Beast is a smart man. He probably predicted the weaponisation of the cure.
Thirdly he's been campaigning for human-mutant peace since ''X-Men 1''.
He's likely in awe because:
- He'd given up hope for a 'cure'
- He's aware of the treatment 'Leech' had been given, but still seemed innocent
- Clearly he still misses being human looking
- He's relieved it's not based on his serum
As for helping Rogue, he probably had too many factors (discussed earlier about Mystique) putting him off.
Professor X is also likely put off because:
- He is for resolving differences, not removing them
- The serum is able to remove his paraplegia; what other side effects might it have?
All this makes Beast's reaction not just plausible, but cromulent.