We study a confined eruption accompanied by an M1.1 flare in solar active region (AR) NOAA 10767 on 2005 May 27, where a pre-eruptive magnetic flux rope was reported in a nonlinear force-free field (NLFFF) extrapolation. The observations show a strong writhing motion of the erupting structure, suggesting that a flux rope was indeed present and converted some of its twist into writhe in the course of the eruption. Using the NLFFF extrapolation, we calculate the twist of the pre-eruptive flux rope and find that it is in very good agreement with thresholds of the helical kink instability found in numerical simulations. We conclude that the activation and rise of the flux rope were triggered and driven by the instability. Using a potential field extrapolation, we also estimate the height distribution of the decay index of the external magnetic field in the AR 1 hr prior to the eruption. We find that the decay index stays below the threshold for the torus instability for a significant height range above the erupting flux rope. This provides a possible explanation for the confinement of the eruption to the low corona.