Not being "hasty in thy spirit to be angry" speaks to me of self-control. We cannot ever develop into victorious, overcoming believers if we do not learn to exercise self-control by managing our emotions, especially the emotion of anger.
Other Bible verses that address the subject of anger...
Proverbs 14:17 He that is soon angry dealeth foolishly: and a man of wicked devices is hated.
Proverbs 16:32 He that is slow to anger is better than the mighty; and he that ruleth his spirit than he that taketh a city.
Proverbs 19:11 The discretion of a man deferreth his anger; and it is his glory to pass over a transgression.
James 1:19 Wherefore, my beloved brethren, let every man be swift to hear, slow to speak, slow to wrath: 20 For the wrath of man worketh not the righteousness of God.
Man's anger does not promote righteousness. Part of righteousness, or the right way of being what God wants us to be, is fulfilling our potential. And we cannot do that unless we restrain our anger.
We all want more than we have in various areas of our lives, but we do not always want to operate within the boundaries of self-control. If we really want to grow spiritually, we must keep our passions under control. That does not mean we have to be perfect or that we can never make mistakes. Although the Holy Spirit will give us power to control our emotions, we may still lose our tempers at times. But as soon as we do, we should immediately confess and repent.
A disciplined, self-controlled life not only requires time and determination, and hard work; it also requires self-denial, but the rewards are worth the effort. By controlling your anger, God will lead you from the darkness into His Light.