Because of increasing terrorist acts in several countries, Changi airport has escalated their security system. There are 400 security cameras spread round the airport to monitor the airport of anything suspicious. Passenger belongings are examined for any weapons, explosives, poisonous items, etc.
For your own safety, don't carry another person's luggage, including your friend's, without knowing for 100% what's inside. You shouldn't leave your belongings behind either because the security guards will confiscate it; this is part of the security procedure.
The security officers are members of Singapore Police Force Airport Division. They routinely patrol in teams of three within the terminals, one of them armed.
Although they've escalated their security, the atmosphere in Changi airport is still welcoming. There are no tensions, they are only upgrading their security for your safety.