Bone deforming too much when attached to vertex group

Hi there,

I have created a character with a rig.
When moving the legs backwards, the lower body is deformed.
In the image you can see in red how the lower body is deformed right now, and in blue how the lower body should look like.

I used this tutorial:
The tutorial is not using weight paint mode, but I still want to change the weight on the body from the leg bone.

In the tutorial the bone were attached by assigning the vertex groups.
how can I change the weight, or prevent the leg bone from deforming the lower body?

Pose it to a position where it’s ugly. Select mesh. Ctrl tab to weight paint mode (or whatever you do, I may be using different startup interface options than you.) Optionally, v to switch to vertex mode, then select some vertices to which you want to limit your effects. Select your leg group on properties/object data/vertex group, look on sidebar/tool, change mode to multiply, weight to 0, strength to 1. Paint until the pose is no longer ugly.