USSR did support North Korea, but being in charge is too strong a statement. Main Soviet interest in Korea was not to have an American ally on it's borders, divided Korea served that purpose just as well as potential Korea unified by the North. From Soviet strategic perspective, war served no purpose, and was in fact detrimental in regards to their European diplomatic initiatives.

As /u/bettinafairchild wrote, the reasons for boycott were independent of the Korean situation, and after US withdrawal from South Korea (likely because of fears that Syngman Rhee might use the situation to attack North and drag US into the war), Soviets didn't expect the return of US forces. Only than did Soviets approve of Kim Il Sungs' plan to attack South, previously he was denied multiple times.

In the end, both Koreas in late 1940s and early 50s are cases of minor allies trying to use their larger allies for their own purposes, of tails wagging the dogs, as Gaddis writes in his excellent Cold War. Kim Il Sung finally got the reluctant approval under the condition that he can provide a swift victory. Further denials risked an independent Northern attack, maybe even supported by newly communist China, and that would put a significant question mark over the world revolutionary leadership of USSR. So, while Soviets did in the end approve the attack, and during the war send some aircraft as help, they did so only reluctantly, while Kim Il Sung successfully used his minor ally status in relation to USSR to advance his own goals.

The Soviets were boycotting the Securitu Council meetings in protest of Taiwan being recognized as China by the UN, rather than mainland China, meaning that Taiwan sent the UN rep, and communist China was frozen out.

Had the Soviets had any inkling of Truman's plan to take advantage of their absence from the Security Council to vote to send troops to Korea, they'd have been there. But they were blindsided.