In terms of criminality, Namibia is very safe. There is some petty crime in the cities so don’t leave valuables visible in your car, and park in secure places overnight. Otherwise there’s nothing to worry about.
Out on the road, the greatest dangers are road accidents (often involving blow-outs or collisions with stray animals rather than other vehicles), breakdowns in isolated places and, very occasionally, wild animals and creepy-crawlies. Drive cautiously, carry spares or the ability to request assistance (eg. a satellite phone for very remote areas) and wear closed shoes at night for the occasional scorpion. Give wild animals space and respect and you shouldn’t have any trouble.