Generally speaking with something like a sword you want to keep each "step" of the pixel the same length. So ideally completely straight or 45 degrees like you have now. But you can also do something like 2,3,4... pixels per step. These positions are generally where you want the animation to "rest" however once you're in motion it isn't so important. Generally you just want to keep the line relatively straight (or not, depending on the animation style)
Use the line tool
If you're using graphicsgale hold Alt to lock the line to a even step. Other pixel software may have a similar function.
In regard to your animation in general, add an anticipation frame. This is a frame before the animation where the character moves in the opposite direction, like "winding up"
Just can say you are right.
Tried to use the line tool instead of doing the lines manually and it works wonders.
Thanks for the hint

Btw., I use Aseprite, it has this function too.
For the animation, CFKaligula made an anticipation frame and it looks really better
Edit: 
Here are some ideas to give the animation a bit more bang. The main way to show that an attack has a lot of force is to add anticipation, the longer the build up, the stronger the attack. From your animation this seemed like quite a fast attack, so i tried not to add too much anticipation.
Moreover, I made the frame where he releases the bolt have more impact by adding the big flash. I also removed 1 frame and added it in this release frame as a smear. If you download the edit you can see that for yourself.
I think your 2 frame idle animation looks fine, if you were to add more frames, you should look into adding secondary motion, like with the cloth between his legs and his hair.
Thanks for your edit !
I see what you mean and the animation has a littlebit more "bang" and is way more fluid

I looked at every frame, but the best edit is your change of the starting hand. It makes the animation more readable.
The idle animation is fine currently for me, since I'm still learning and try not to be too much of a perfectionist, as I'm often too critical to myself and my pictures :/
Btw., this shouldn't be a dude, it was supposed to be a female ^^
Any ideas how I can change that with shadows :/ ?
//EDIT:
Played a littlebit with the shadows and colors

Hope it is more readable in this image that she is a woman ?
