Muscles posted...
If you pull it you are at least partially to blame for the single person's death, if you don't pull it you share none of the blame.
Debatable (which is the point), because you could have acted and saved five people's lives. Your inaction is as (partially) responsible for their deaths as your action would make you (partially) responsible for the death of the single person if you did pull the lever.
I personally don't find the trolley problem in its most basic form to be a particularly difficult one, because it's a relatively simple equation: act and one person dies, don't act and five people die. From a cold, dispassionate perspective, the former is eminently preferable to the latter, as it causes the least amount of suffering.
There are much more difficult variants of the trolley problem out there. For instance, what if the one person is a child, and the group of people all have terminal cancer? What if the one person is an innocent civilian, but the five are all murderers? You start getting into discussions on the subjective and comparative values of different lives, where the answer is not nearly so clear-cut.