why the corners do not go perfectly straight ?
Slop. Basically there’s movement in the system somewhere and your head cannot stop and turn 90degrees without continuing in the direction it was going a little.
You can tune this out (slows the movement down towards turns) and make sure everything is a tight as is should be to reduce it.
Google backlash reduction etc…
The worst is on just one corner. That looks like where the perimeter loop starts and stops. You might find help in tuning your machine better (make sure there’s no backlash, belt tightness is good, adding some jerk and acceleration), adjusting slicer parameters might help. Maybe increase retract speed and distance.
You also appear to be over-extruding.
The blobs at the end of infill lines are indicative of this. It will also influence the quality of your corners.
I’m fighting this too. If I lower the extrusion rate, then in areas with lots of short runs, it grossly under extruded. Turn it up to deal with short runs and corners bulge, infill overflows, etc.
You are probably using kisslicer version like 1.1? Try downloading V1.5 beta from site, it has implemented start/stop control and also way more control about retraction and a bunch of other improvements, think you’ll like it 
Also, try boost up seam hiding, you have it on 0.
Seam hiding should help you, it looks like that’s where your layers are starting.
Use fillet Command at the corners
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