Why must I take abs of filter frequency response?
Frequency response has two parts: amplitude response and the phase response. Both of these are represented as a complex signal when you get the response from freqz. In order to plot the amplitude response you need to use abs. Otherwise I doubt it only shows you the real part which I think is what you see in the first figure. Note that when dealing with the amplitude response, in most cases you will do a further step and convert it to $\textrm{dB}$ scale.