In itself, anger is not a sin, because even God can be angry (Deut. 9:8, 20; Ps. 2:12). Several times in the Old Testament the phrase appears, “the anger of the Lord” (Num. 25:4; Jer. 4:8; 12:13).
The holy anger of God is a part of His judgment against sin, as illustrated in our Lord’s anger when He cleansed the temple (Matt. 21:12-13).
The Bible often speaks of anger “being kindled” (Gen. 30:2;