A human mind not giving due consideration to the effects of granting arbitrarily high intelligence to an agent with simplistic morality counter to human morality.

From there it's a sequence of steps that would show up in a thorough root cause analysis ("humanity, the postmortem") where the agent capitalizes on existing abilities to gain more abilities until murder is available to it. It would likely start small with things like noticing the effects of stress or tiredness or confusion on human opponents and seeking to exploit those advantages by predicting or causing them, requiring more access to the real world not entirely represented by a chess board.