For the most part, a myth. There is not enough evidence to suggest a relationship between exogenous testosterone usage and anger, and a fair bit of evidence to say that no relationship exists.
Injections of 200mg/week testosterone note a positive trend towards increased sexual behaviour, although non-significant. Another study with the same dosages of testosterone (as enanthate) found increases in sexual desire, but not necessarily sexual action; there was no increase in self-reported aggression either. Men who are hypogonadic before testosterone administration appear to be more susceptible to the increase in sexuality.
One study on healthy men and those with reduced testosterone levels subject to testosterone enanthate (200mg weekly, twice that for hypogonadism) do not report increased aggression. However, when controlled for measures of impulsivity a relationship is seen. A study conducted with 600mg weekly and surveys given to both active/sedentary men and their spouses reported no relation between testosterone and anger.
Other studies note (via self-report) slight increases in agitation and aggression at 200mg weekly.
Differences in results may be due to the given survey's word structure.
Hypogonadic men (men with reduced testosterone levels) show an increase in subjective well-being and mood when having their testosterone levels replenished whereas eugonadic men (normal test levels) show no treatment effect. Other studies note slight increases in subject well-being across the board.
200mg testosterone enanthate weekly in healthy men (or biweekly in hypogonadism) seem to increase verbal fluency while decreasing spatial abilities and in hypogonadic men, these effects are somewhat contested though, as evidence by a study in elderly men showing seemingly opposite effects.