Right, while being technical on definitions, per Geneva Conventions:
The Protocol Additional GC 1977 (APGC77) provides the most widely accepted international definition of a mercenary, though not endorsed by some countries, including the United States. The Protocol Additional to the Geneva Conventions of 12 August 1949, and relating to the Protection of Victims of International Armed Conflicts, (Protocol I), 8 June 1977 states:
Art 47. Mercenaries
1. A mercenary shall not have the right to be a combatant or a prisoner of war.
2. A mercenary is any person who:
(a) is especially recruited locally or abroad in order to fight in an armed conflict;
(b) does, in fact, take a direct part in the hostilities;
(c) is motivated to take part in the hostilities essentially by the desire for private gain and, in fact, is promised, by or on behalf of a Party to the conflict, material compensation substantially in excess of that promised or paid to combatants of similar ranks and functions in the armed forces of that Party;
(d) is neither a national of a Party to the conflict nor a resident of territory controlled by a Party to the conflict;
(e) is not a member of the armed forces of a Party to the conflict; and
(f) has not been sent by a State which is not a Party to the conflict on official duty as a member of its armed forces.
All the criteria (a f) must be met, according to the Geneva Convention, for a combatant to be described as a mercenary.
Given that the recruitment of Pakistanis into the Bahraini security forces was a program put in place long before any conflict began, I do not see how it meets the definition of 'mercenary' per the Geneva conventions.
Also see below additional points from the Geneva Conventions applicable to the Bahraini attacks on Pakistani civilians:
Articles 51 and 54 outlaw indiscriminate attacks on civilian populations, and destruction of food, water, and other materials needed for survival. Indiscriminate attacks include directly attacking civilian (non-military) targets, but also using technology such as biological weapons, nuclear weapons and land mines, whose scope of destruction cannot be limited.[5] A total war that does not distinguish between civilian and military targets is considered a war crime.