There is nothing wrong with the code.

It is because you have too large value for k1,k2

The resistor force is assume to be F=K1*V+K2*V^2 and the mass is assumed to be 1.
K1 and K2 has to be much smaller than 1.
for k2=0.1 it is not an object moving in air. It is an object moving in densed fluid.