First Penguin edition, 1963.
From the back cover:
Anything could happen on a frontier planet, and Pyrrus was a shining example. Every form of non-human life, flora and fauna, mutation upon mutation, had combined with the impossible weather, the earthquakes, and the volcanoes to become a deadly weapon ... for the extermination of man. The Pyrran settlers had developed superhuman strength and reactions and had imported the most modern aids to warfare, but the odds were too heavy against them. Jason dinAlt, interplanetary gambler, had allowed and even helped himself to get sucked into the fray without realizing that to relax your guard for a split second was one split second too many. Appalled by the conditions, he fought a single-handed war, not only with the planet but against its murder-minded people, to stop the carnage and bring peace to the war-torn outpost. To say it was hard would be a galactic understatement.
What Jason discovers is that the planet's life-forms have all evolved psi powers: their hostility and aggression only reflect the feelings of the humans they meet.