Foreign Policy published on 3 April 2015 an analysis titled "Al-Shabab Crosses the Rubicon" by Bronwyn Bruton, Atlantic Council in Washington. You can access the article by registering for free.
The author aruges that the recent attack by al-Shabaab on Garissa University in Kenya represents a final point in al-Shabaab's long evolution from a populist resistance movement into a full-blown, international terrorist organization that may be trying to emulate Boko Haram and even establishing links with the Islamic State.
While I agree there have been factions within al-Shabaab from the beginning, to consider it once a "populist resistance movement" gives it a credibility that it never deserved. It has always relied on draconian measures and terrorist tactics; the tactics have just become more draconian and offensive with the passage of time.