Then again you could simply just thrust straight at the opponent and kill him before his sword falls
You could: this is why foil fencing, for instance, makes it a rule that if your opponent begins an attack, the attack MUST fail (eg be blocked or miss) before you can score on the counter. Of course that's good for sport but the fact that the rule is necessary shows that being the first to strike would normally be by far the best in combat.

Also in foil (never done sabre but would like to) you must parry with the bottom half of the blade nearest the grip. You simply can't parry with the tip, the leverage against you is too great. Of course in foil the blade you are parrying has next to nil momentum being so light.

The point about thrusting is absolutely right. Its much quicker than a swing, and I suspect more deadly, since you are heading directly for vital organs. Your en garde position is safer too, since your point is aimed directly at the target area at all times. If he is fast, even an unarmed man stands some chance against a man armed with a slashing weapon, as there is a moment when he must draw it back to strike, and you can step in. I remember practicing this in Ju Jitsu against opponents with "truncheons" and all I would say is that, after practice, its reasonably easy to do reliably in the dojo when your opponent is a friend with a plastic bottle ! In principle it would be just as easy against a barbarian with a broadsword but I'm glad I never put that to the test...