Well Dark Side was never weak. If u'll try to be neutral , think about it, Dark Side bring it's user/follower power over man whatever it take's. One Sith Lord like Palpatine was able to destroy a whole regime of republic and become a single Emperor ruler, how weak is that. And When weak Vader let his feelings out he just show that he never had enough balls to be a Sith to have unlimited power over man. But yes i can agree that from the beginning his intention to follow dark side was in the name of the good ( to save padme) so he basicly was forced into d.s. he didn't joined it totally by his own will(therefor he was weak from the beginning). Plus do not forget the sandman village that he slaughtered i believe that's where he 1st felt the real meaning of pain of losing mother and hate and anger to the sand people.
P.S:He was not good(bad) enough to be a sith.
To be fair though it was the ignorance of the Jedi that led tho their downfall. Remember that scene in the Episode 1 I think it was where the Jedi were first told of a Sith lord? Mace Windu denied the possibility right away. Most of the Jedi felt the same way, like how in Episode 2 that librarian lady said that if something wasn't in the archives it didn't exist. They had been in power so long they became arrogant.
And that's just what we saw in the movies, Palpatine's plans was already well in motion by that time (the Naboo blockade was already active). He started his plan right under their noses, if they hadn't been blinded by their own success they might have seen things in time.
Anakin joined the Dark Side for selfish reasons, purely because he feared losing Padme and experiencing grief again. He was not forced, manipulated, but not forced. He had a choice all along. He could have killed Sidious, he instead chose to join him.
He also didn't HAVE to kill everyone in the camp. He killed out of anger, and the anger gave him strength.
Also don't forget his lust for power. In episode two he spoke of finding a way to prevent death, he wanted to be the most powerful Jedi.
There was a darker side to Anakin for a while before he joined up, and I think to call his emotions weakness is ridiculous. Selfishness as a result of those emotions are what drove him to darkness. Wanting to save Padme was not pure or noble, she would have met her fate gladly. He pained her and broke her heart in his persuit of power.