The weight is not making it flip over. When loaded to the max they seem to struggle at lift off but all of mine still lift off upwards. If completely overloaded they seem to lift then settle.
Your video reminds me of first flight attempts that had something reversed. Motors, esc connections or servo channels are the most common reversals.
Are you using the KK 2.0 board.? Why no I gain? Are the roll and pitch linked?
To check the system out. Take the props off. Arm the quad. pick it up stay clear of the motors and bring the throttle up. Lean it forward and the front motors should speed up, lean it back and the rear motors should speed up. Repeat this with left and right roll then move it to lower one motor at a time. The motor that is the lowest should speed up. Lastly holding it level move the pitch stick forward, the rear motors should speed up. Pitch back = front motors speed up. Stick right = left motors faster and stick left = right motors faster. Please do not try this with the props on.
If all this checks out it should be able to take off. Have you calibrated the sensors.
Let us know what controller you are using and if these checks show that all is working correctly and we can go from there.
Be safe those carbon props are very stiff.