I was just reading a post in explainlikeimfive why guillotine blades are slanted. There were a lot of posts about tomatoes and how going at an angle through the neck is easier than going straight down. I do not see how going at an angle could decrease resistance. I am not a physicist, but it seems like going at an angle only increases the length of the cut while not decreasing any amount of resistance. (Mods, if this is out of place, go ahead and remove it. This is just the only sub I could think of that might be able to give me a real answer and not anecdotal evidence from cooks about cutting tomatoes.)
You mention resistance. This is correct. With a slanted blade, the point of resistance is in a smaller area when it first arrives at the person's head. This means that the initial impact does not slow the blade as much as it would if the blade were straightened. The reason this is important is that it makes it more unlikely to get lodged in the middle of the neck or hardly entering it at all. I mean, would you wanna be the guy to have to break off the rest of the head from the blade, especially if the dying person is still screaming?