If you don't know the rules, here they are.
In the Red Corner: Red indeed! 10 Sons of Sanguinius stand tall against the Imperium's foes, chainsword in hand and jump pack fully charged. 2 have traded their chainsword for flamers.
In the Blue Corner: 11 slippery Genestealers lurk about. Their talons drip with toxic ooze.
The Blood Angels have the advantage for the assault here - jump packs trump Fleet.
The gates open!
The Blood Angels resist the red thirst. Never mind the advantage +2 on the roll to assault - the Nids win the roll 6-3 in style. Furthermore the Blood Angels fail the 4+ roll required to start in cover. This could get nasty folks...
Eschewing ranges combat, the 11 'stealers leap into combat and use their Initiative 6 to carve into the flying Marines. In a flurry of dice rolls, including rerolling all failed wounds, they inflict 13 normal wounds and 5 rending wounds on the poor Angels. Wound allocation trickery softens the rends to 3 kills - both flamers get double rends allocated. 3 other marines fail their saves - including the Sergeant with power fist.
The 4 remaining Assault Marines land 5 hits, 3 of which wound, converting to 2 kills. The marines flee the assault, but are not caught despite a significant Initiative advantage, so the battle continues.
Round 2 and the Genestealers only manage 7 wounds, with no rending 6s. 2 Marines fail their saves, leaving only 2 to to swing back. They get 3 hits and manage to convert this to 2 deal 'stealers. The round is a draw.
Now reduced to 7 models, the Tyranids inflict a healthy 6 wounds on the remaining couple of marines. They die.
Aftermath: The Blood Angels had nothing to threaten their opponents, especially when faced with such a lethal first turn effort on the Genestealers' part. Even if they had won the charge, without Furious Assault and Feel no Pain granted by a Sanguinary Priest in proximity they are a shadow of their normal selves.