In 1948 Walter Evans, a young aircraft engineer, asked a simple question: as you turn up a feedback controller's gain, where do the closed-loop poles go? His root locus answers it graphically — every closed-loop pole traces a smooth branch through the complex plane as the gain rises from zero to infinity.
The branches always start on the open-loop poles (the crosses) and end on the zeros or run off to infinity, obeying a pure angle condition; the gain then fixes exactly where on a branch each pole sits, by a magnitude condition. Pole location is behavior: real poles give a smooth climb, a complex pair sets the ringing frequency and its distance from the imaginary axis sets how fast it decays.
The imaginary axis is the cliff edge. A pole to its left decays; a pole on it oscillates forever; a pole to its right blows up. Find the gain that lands a pole exactly on that axis and you have found, to the decimal, the gain at which the whole system loses stability.
The simulation stopped unexpectedly — the lesson continues without it. You can move on; nothing you did was wrong.