It all comes down to chance: skill can decrease the likelihood you'll die, but it won't remove it entirely. You can survive a fight against fifty men one day, and then be killed by two the next. More than that, Skjor was careless and quite literally charged into a fortified garrison alone. He was surrounded and in close quarters, and both of these things usually mean death. Just because he survived it once doesn't mean he can survive it every single time. As for Kodlak, he was taken by surprise, and perhaps fatally wounded before he could start fighting off his attackers.