The comparative form of the adjective "bad" is "worse." Unlike regular adjectives that tack on "-er" (like "tall" to "taller"), "bad" is irregular, so "badder" isn’t correct. Use "worse" to compare two things—e.g., “This test was bad, but the last one was worse.” It’s simple once you know it!
Further reading: What is the superlative form of the adjective “bad”?