If I understand it correctly, John Kerry voted for the most recent Iraq War resolution to put the United States in a stronger bargaining position. That is, he thought that if the UN believed the US was going to go to war anyway, it would step in with a more aggressive sanction policy which would thereby prevent war.
Now to me, that rationale is utterly retarded.
First of all, it implies a great deal of trust in George W. Bush. It implies that John Kerry actually thought that it would be possible for the UN to stop Bush's drive to war, especially after that war had been given the supposed approval of the American people. That honestly does not give me much faith in Kerry's ability to judge character. It also means that Kerry failed to really exercise any legislative muscle. How hard would it be for Congress to say "we vote for war BUT you have to come back to use to get it authorized again in two weeks" or whatever. Kerry didn't even try.
Second, and more seriously, it shows a shocking attitude towards the United Nations. Kerry's rationale boils down to: "Give us what we want or we'll do something worse!" It's like saying "well I'm against suicide, but I gave that depressed man a gun because I thought it would make the police take him more seriously".
Honestly, you don't treat organizations you respect like that. If you really care about multilateralism then you don't go into the UN with your mind made up and say "ok, now you support us, or else". If you respect a democratic organization, then you don't go start your diplomatic process by voting for an extreme resolution in the hopes that you can force them to adopt a program you want.
Kerry's position shows as much disregard for the power of diplomacy as Bush's did. Yeah Kerry may not have expected Bush to "fuck things up" as badly as he did, but what can we expect from President Kerry? Will we really get a much healthier attitude towards the UN from him?