I completly agree with belaran on the "No" for the bodyguard revealing themselves...
I also LOVE the suggestion of making him possessed by a powerful Insect Spirit (Aurelius is a great pick for that for the reasons given). If you go this route, it's revealed to them when the Johnson tries to 'initiate' them into the fold, as it were, as their reward for their service.
Twitchy would of course be revealing this after overtaking the group- bodyguard is already on route. (you can make this scene really creepy as he gets twitchier and twitchier, slowly sloughing off his human guise to reveal the insect underneath...

As for 'why' because the target is in a vehicle on-route to the office or something, and It's specially armored, and to snipe the target at range effectively would require this laser McGuffin. This also puts the whole thing on a timer they have to race against.
Better yet, as to why they would want to save the big-wig? Ultimately they want to infiltrate Ares with insects. The McGuffin is a high-powered, focused microwave transmitter. It'll slowly fry the vehicle's systems, bricking it and locking everyone inside. If they don't stop the Johnson, well... you get the idea.
If you don't want to go for the insect spirit idea, it could also be competing Exec's. Same scenario as above, using the laser snipe idea. Why to save them? Because it's all revealed to them by a party that just approached them with a counter-offer...
Or
Maybe the 'actual' employer is Aurelius, and suspects another one of the board members is actually a bug spirit. The Bodyguard is a hit-man that was paid (very well) to take out the bug, and your team was hired to be the patsies to get the laser (so they'd be associated with the hit, not the actual assassin- that would also explain how the bodyguard knew about the prototype to begin with). The prototype is also to ensure that it will not only pierce the vehicle, but also kill the bug.
As they're leaving, having given the McGuffin their employer, one of them gets a call... from Damien Knight (have a receptionist make the call and have them wait to some crappy music while they try to decide whether or not they believe it's actually him calling). Once he gets on, he make it clear that he is aware your team was the one who stole HIS prototype. He's also aware of who hired the hit-man, and who the target is (though he won't reveal it was Aurelius).
He then informs your team that everything that's happening is a 'misunderstanding' that he is fixing (the board member isn't possessed- Aurelius is just paranoid). if they do not stop the hit-man and retrieve HIS weapon, life will become very, very difficult... and short, for them.
So, you'll have them fighting Ares forces to get back a weapon to return it to Ares... and a rather determined hit-man.
Me? I'd throw in the other Shadowrun group that has tracked them down (or if they're all dead, another group that that original employer of that job hired to hunt you down- that fact that the bodyguard said he hired that team was a lie) that shows up while your team is trying to get it back themselves, just confusing the issue.

Just throwing a few more ideas out there!
Crunch~