Decision-making in many-worlds is exactly identical to ordinary decision-making. You weight the utility of possible outcomes by their measure, and add them up into an expected utility. The bad stuff in one of those outcomes only feels more important when you phrase it in terms of many-worlds, because a certainty of small bad stuff often feels worse than a chance of big bad stuff, even when the expected utility is the same.
Decision-making in many-worlds is exactly identical to ordinary decision-making. You weight the utility of possible outcomes by their measure, and add them up into an expected utility. The bad stuff in one of those outcomes only feels more important when you phrase it in terms of many-worlds, because a certainty of small bad stuff often feels worse than a chance of big bad stuff, even when the expected utility is the same.