How can I use weight painting correctly in Blender? Before when I was weight painting, I noticed that after changing the weight of a bone some other bones would be dramatically changed as well. When I try to undo the unexpected weights the whole mesh becomes broken. Now, when I try to subtract the weight, instead of subtracting it adds. Can someone please help me? I've been spending hours on this!

This is my blend file:
http://pasteall.org/blend/index.php?id=50825
The reason why you have this issue is that you have Auto Normalize enabled:



Either try to understand how to work with it (because it's honestly useful), either disable it.