Instead of recoiling when he was grabbed, L allowed it, not resisting in the slightest. It allowed him to get close enough to Kain to wrap this match up neatly.
Kain knew that he wasn't any great shakes at magic. Kain thought that L's arrogance would be his downfall. L managed to stab the syringe into Kain's thigh and empty it an instant before he was punched, harder than he'd thought even Kain was capable of, in the stomach. He could feel two of his ribs bend and snap under the impact, but Kain would be hurting worse in little more than a minute. He should have known better than to fight with a pit viper, after all.