Tip: Build paths when behind and ahead are actually the opposite of what most think.

Short and sweet: When behind, you build damage on items that give you small defenses.

When ahead: You build defense on items that give you small offenses.

The reasoning behind this is solid.

When youre behind, building tank is useless. Youre behind in gold, levels, and damage. Tank shred is a huge part of the meta right now, so building tank not only gives you less stats for the gold you spent, it also means youre a sitting duck. You have virtually nothing to fight back with. Most situations, you either get obliterated anyway, or are ignored and the important members of your team are whiped.

Building damage lets you fight back. Assuming you have good positioning, most of the time you wont be in a position to die unless you take someone with you. When you're behind, 1 for 1 exchanges do more for you than your opponent. It gives you gold, exp, and gives your team a chance to take advantage, while at the same time shutting down the target, giving a bounty to your team and shrinking their lead.

Building damage lets you fight back. Now this doesn't mean if you are 1/6 as jinx you should be sitting on the front lines with an IE, it means you should focus on surviving the fight and attacking when you can, if you take your opportunities, you should at least make a trade or two, if not come out on top.

When youre ahead, you want to build tank items. People who are behind cannot afford to build penetration items. Building damage when you are head leads to a cocky persona and increases the chances of you dying, thus shrinking your lead. Building tank means that since youre already unstoppable with your 6/1 score that you will continue to be unstoppable because they cant kill you. Your levels and small advantages from tank items damage (like sunfire or dead mans for example) provide damage boosts to make sure you aren't lacking in trades. Simply put, if you die less, you do more, and win more. Since youre tanky, you can afford to do objectives like dragon and baron more often with fewer resources, since you are a big soak of its damage. This gets you more damage and your team a bigger lead

This doesn't mean you should be jumping in 1v5 as a 6/1 darius, this will diminish your lead. But grouping up with your tankiness will assure you will take 3 or 4 people out with you while you mow them down and you will 100% come out on top if you take advantage of the lead.

Leads are unfortunately an unprecedented part of the meta right now. Games can be decided as early as 7 minutes if you aren't careful. If you follow the advice, building damage when behind will help bring you back in the game, while building tank when ahead will help push your lead.

Hope this helped.