According to the Nyquist criterion this system is stable but when I apply an impulse I get an unstable response, why?
Review the Wikipedia article, or your textbook.

The number of times the plot can loop around the critical point (-1 for open-loop plots, 0 for closed-loop) is equal to the number of unstable poles minus the number of unstable zeros.  This is because you are counting the number of poles minus the number of zeros that are encircled by traveling up the \$j\omega\$ axis, then around the entire right-half plane.

So your Nyquist plot is actually predicting an unstable system, which is exactly what you have.