"If there is competition..."

If the gas stations collude to set prices, that is by definition not competition. But as you also said, "one of them can cheat..." This is not a hypothetical but an inevitable expectation. Cartels always have cheaters which ruin the party. The temptation to lower the price just slightly to gain more market share (yes greed) is too strong and undermines the cartel. OPEC is a good example. Members agree to cut production to raise the price of oil, but after a short time there is ALWAYS at least one member who raises production over the agreed point to make more money. The rest then follow suit.

Monopolies are a different story since they have no competitors at all and thus don't have market pressures on them. They will charge more and provide inferior products or services. We're both old enough to remember when Ma Bell had a monopoly on America's phone service. Once it was broken up phone service vastly improved. Voice mail, call waiting, wireless phones, cell phones, cheaper rates, less service interruption, etc all followed the break up of Ma Bell.

I think we can agree that monopolies rig the market. So do cartels at least temporarily. Where you and I will never agree is that the market is rigged when there is competition.