Those fish are not tetras (although I have frequently seen them labelled as such in stores
), they are Harlequin Rasbora, specifically Trigonostigma heteromorpha. You can read about them in our profiles, just click on the shaded name to see the profile. They behave much the same as tetras (which are characins), so I guess stores think more people will "recognize" the name tetra.
These fish are about as docile as fish can be. All fish are inclined to go after one that is sick/injured, depending upon this and that, so it may be this. This species (and all rasbora) are shoaling fish, just like true tetra, so they need to be in a group of at least 6. Aggression is a common result of being kept in too small a group. Also, shoaling fish will sometimes turn aggressive when they are in too small a space. What is the tank size?