While strictly not negative, there are some that could imply you aren't playing your best or are playing the wrong class:
Nemesis - Get five revenge kills. - Means that you've had five people dominating you. However, this doesn't mean all at once, so it's not that negative.
Engineer - Death Metal - Pick up 10,000 waste metal from pieces of destroyed buildings. - To get this, a lot of your buildings (or your teammates/enemies buildings) have to be destroyed. Likely, this is because your sentry or dispenser got destroyed, and you're picking up the pieces for metal to build new ones.
Another class of achievements are ones where, to get them, you have to arguably play your class wrong:
Spy - Die Another Way - Kill a Sniper after your backstab breaks his Razorback. - Why are you backstabbing Snipers that have Razorbacks?
Pyro - Plan B - Kill 10 enemies while you're both underwater. - Pyro + Water = Bad
Heavy - Photostroika - Provide an enemy with a freezecam shot of you taunting while invulnerable. - AKA wasting the rest of the Uber.
Medic - Does It Hurt When I Do This? - Kill 50 Scouts with your syringe gun. - There are going to be some times when you're fighting Scouts, but if you're going out of your way to fight enemies, you're doing it wrong.
Medic - Peer Review - Kill 10 Medics with your bone saw. - See above, but add in the fact that you're not even using a distance weapon for this one. At least they cut back on the number of kills for this one.
Any taunting achievements that aren't taunt kill achievements. There is at least one per class.
Currently the TF2 editor is not able to do this, maybe in the future but for now you'll need an external editor.
If you are using windows, you should have the Windows Movie Maker installed already, though it is not a very good editor.
If you just want to stitch together 2 videos, you can do that with the YouTube Video Editor: http://www.youtube.com/editor
Best Answer
I have auto-reload disabled in my spy.cfg because when I'm using the Ambassador, the reload animation obscures the reticule, which slows me down when I'm trying to set up the second headshot needed to finish off some of the tougher classes.
In your spy.cfg file (Steam\steamapps\\team fortress 2\tf\cfg\spy.cfg):
In all the other classes:
(By the way, you're right, viewmodels off would also solve this problem, I just like them better on.)