Adding the word spree here means that it's not just a one-off bombing but it's like a series of bombings. The bomber is going crazy and is bombing places one after the other. This word is used often in the corporate world when a businessman is buying or acquiring new businesses. We say he is on a shopping spree.

a period of excessive or unrestrained activity ... in this case, a series of bombings New Oxford American Dictionary spree | spri | noun a spell or sustained period of unrestrained activity of a particular kind: he went on a six-month crime spree | a shopping spree. Collins Online Dictionary https://www.collinsdictionary.com/dictionary/english/spree countable noun [usually noun NOUN] If you spend a period of time doing something in an excessive way, you can say that you are going on a particular kind of spree. Some Americans went on a spending spree in December to beat the new tax. Longman Dictionary of Contemporary English https://www.ldoceonline.com/dictionary/spree a short period of time when you do a lot of one activity, especially spending money or drinking alcohol on a spree They went on a drinking spree. a shopping spree Examples from the Corpus spree • Then, against the background of bitter, violent parental quarrels, Chester began a spree of delinquency. • Some stores were bright on reports of an eleventh-hour Christmas buying spree. • He had been sentenced after going on a two-month crime spree.