How did the world war 1 become so bad?

It depends on what you mean by bad.

• So many countries were involved because of treaties that were signed well before the outbreak of war. Therefore making it inevitable that so many countries would be involved.
• In terms of deaths, the trenches were a system which had not been used before, this was the first time where warfare as we know it was used. Before this battles had been large scale engagements in ranks, such as the Nepolionic wars. Therefore the generals were inexperienced and so many lives were wasted by 'going over the top'.
• The trenches were first built by the Germans because the British and French counter attack was pushing them back at a very rapid rate. Thus causing the aforementioned situation.