This is just silly. Against his better judgment Roger steps aside when Mina starts talking. First Wrex, confirming the man's (probably quite justified) racial bias, then Mina, confirming to him that their group felt superior to the small-town folk? One can only smooth over so much before begins to look like desperation borne from keeping bad company, and Roger isn't about to confirm the poor sod's third assumption. Obviously the others had their own ideas on how to handle this, and even if they (or at least Mina and Wrex) were not doing a particularly fine job, it would be helpful to observe how they handle things for future reference. It always rubs Roger the wrong way when people use the wrong tool for the job anyway, and Roger quite well qualifies as the right tool for this job.

Besides, there was nothing in Roger's tone to suggest any hostility or arrogance, so if this escalates then the guard quite well brought it on himself.