Factual error: When the police chief is looking at Danny's police file just before calling his father, you can see a few different phone numbers listed. These numbers are all using 213 as the area code which is for Los Angeles, CA. Given that the film takes place in New York City and that the addresses listed on the same police file are NYC addresses, the phone numbers should be using 212 area codes. (00:28:22)
Factual error: April interviews (live) the police Chief and then tries to take the train back home. The subway station shows that it's the City Hall stop, and she just finished her interview with the guy, which happened mid-afternoon as you can see by the clock in the corridor. Despite being New York in the middle of the day, there's not a soul in the station, nobody got off the train she just missed, nobody around at all on any of the multiple platforms. It's a trope good for a scene set at 3 AM, not 5 PM. (00:19:25)
Answer: Most likely to kill Shredder and make sure that there would be no way to come back but, as seen in the sequel, Shredder does return.