I haven't read deeply into either. What I know at a glance:
1 - Alton Sterling
This guy was selling CDs in the parking lot outside a convenience store. Initial report was that police responded to a anonymous call reporting him as a threat. Looks like there's more detailed information emerging today, claiming that the call was made by a homeless man who had been harassing Alton persistently to beg for money. Alton was carrying a gun. I haven't seen it definitively stated, but every mention of the incident points out that it's an open carry state, with the implication that he was carrying a firearm legally. Supposedly, he flashed his gun at the homeless man and insisted that he be left alone, at which point homeless dude calls 911 and reports a black man brandishing a gun. As far as I can tell at this point, police showed up and there was absolutely no conflict or anything suspect apparently happening.
But police proceeded to violently arrest Alton anyway, throwing him to the ground and restraining his limbs. Clearest details are from the video, which show Alton on his back, with officers kneeling on him. He's squirming, but it doesn't appear to be serious resistance beyond natural reflex to having your joints ground into concrete. One officer is feeling in Alton's pockets and announces that he has a gun. Not sure if it's one or both officers, but the one closest to the camera at least immediately pulls his gun and unloads point blank into Alton's chest.
2 - Philando Castile
Officer pulls over Philando's vehicle due to a broken tail light, and approaches the window. Philando informs the officer that he legally has a licensed firearm stored in the vehicle, and that he is going to reach for his ID. As soon as Philando begins to reach into his pocket, the officer fires into him 4 times. Philando's girlfriend immediately begins a livestream video to Facebook and describes in detail what just happened, and begins to narrate what's happening for several minutes, as the officer stands there with his gun pointed at the guy watching him die with zero fucks given. I admire the hell out of that woman's composure in reacting to the situation. This one's likely to be a lot of trouble, as I'm seeing headlines that Castile was a highly respected public school employee, regarded as a great role model to hundreds of kids.