How would you explain the seventh stanza of "The Inchcape Rock" by Robert Southey?

In the seventh stanza, we learn that Sir Ralph the Rover "felt the cheering power of spring." The good weather improves his mood, and he's happy to enjoy a sunny day on the seas. In fact, the spring weather makes Sir Ralph so happy that he's inspired to whistle and to sing.
In the next two lines, we learn that Sir Ralph's heart is "mirthful to excess." This just means that he is overwhelmed with happiness in his present condition. However, we are warned that this mirth is of a wicked quality. The line "the Rover's mirth was wickedness" possibly refers to the diabolical plans Sir Ralph has in mind. In other words, Sir Ralph is only extraordinarily happy when he's planning some wicked action; his chief purpose in life seems to be to bring suffering and sorrow on all he meets.
The eighth and ninth stanzas support this interpretation because we see that Sir Ralph's chief aim is to "plague the Abbot of Aberbrothok" and to cause the deaths of unsuspecting sailors. To support his fiendish goals, he's more than happy to cut the bell off the Inchcape Float so that no one will suspect they've crashed into the Inchcape Rock until it's too late.