If you happen to die while playing (which could happen quite a lot depending on your difficulty level and skull settings) you will LOSE points.
The base penalty for dying is -25 points. This value will get multiplied based on your difficulty setting though. So, every time you die while playing Legendary will actually cost you 100 points. If you betray one of your teammates you will also incur a point penalty. The baseline value on Normal difficulty for a betrayal is -50 points.

Timeliness
The last major factor that will impact your final campaign score is the amount of time you take to finish the mission. At the start of each mission (from the very beginning, not an insertion point), you will notice a timer above the scoreboard along with a multiplier value. Initially, this will read as “3x.” However, as time goes by, this will eventually decay to 2.5x, 2x, 1.5x and then finally 1x (which is zero bonus). Unlike the multipliers listed above, the time bonus is not added until the very end – when the mission is finished. This is huge because it can drastically alter your score and it’s added on top of what you already earned through style and skulls. It will be challenging to survive, earn points with stylish kills AND complete the level fast enough to earn a 3x bonus but it can be done. As noted at the beginning of this guide, this is not the recommended way to play through the game your first time since “speed running” will certainly detract from the story we’re trying to tell.

Finish the Score
As you can tell, there’s a lot more that goes into your final score than just being a pretty good player. The highest possible scores will only be possible when someone beats a mission on Legendary difficulty, quickly, killing enemies with style AND having as many skulls enabled as possible. To get the true maximum score, you’d have to activate all nine golden skulls – if you can even find them all. And then actually finish a mission. Good luck with that!

Bonus Campaign Scoring Tips & Tricks
Over the course of testing the game, a few of us around the office have developed some ingenious and devious techniques to improve our own skulls and sabotage our teammates.

• Oops! Nothing makes your friends score decrease faster than a bunch of unplanned betrayals. The easiest way to do this is to charge into the fray, preferably right after your teammate chucks a plasma grenade. Sit on top of it for instant griefing and knock your partner down a few notches.

• Kill Steal! Another tried and true tactic is to hang back with a BR or Carbine and let your teammates soften up the enemies while you steal all the kills. Remember, the points only go to the person who lands the final killing blow.

• The Waiting Game! If a teammate dies during co-op play, they won’t be able to respawn until the surviving players are out of combat. You could use this to your advantage by continuing to fight, and boosting your score, while your dead buddies are watching helplessly thought the death-cam.

• Achievements! All of the scoring Achievements are set just high enough where you will almost certainly not earn the achievement unless you have at least one skull turned on (and are playing on the Bungie recommended difficulty of Heroic). The easiest way to hit these score totals is to turn on one or two skulls and play the game on Heroic. In most cases, as long as you don’t die too much and you throw in some headshots and style kills, you should be well on your way to glory.
  

• Smack! Lukems has a particularly devious tactic that involves delivering a quick melee attack to your teammate right as they head into a fray, which immediately drops their shields, making them incredibly vulnerable to death… which means you get to clean up all the kills and nab all the points for yourself.