Thank you for the additional information.
Unfortunately, it is unlikely that you could successfully sue the police department. Officers have qualified immunity from lawsuits like this...basically, they are protected from lawsuits for discretionary choices they make in the line of duty. Prosecutors are also immune from lawsuits for the decisions they make in the scope of their duty. Unfortunately, people are often aggrieved by the charging decisions that are made, and so the government put this immunity in place to avoid having the police and prosecutors sued all the time by people who don't like their decisions. I understand that this is frustrating and feels unfair to you, but unfortunately the government put these immunities in place to block these types of lawsuits. There's just no way to force the police to charge someone, or to force the prosecutor's office to try someone. They are granted the discretion by law to make these decisions. I'm sorry that they won't see the case through, but there is no legal mechanism to force them to do it.
The only thing I can suggest is that you investigate the possibility of filing a wrongful death lawsuit against the perpetrator. The standard of proof is lower in a civil case than in a criminal
case, so even if the police/prosecution don't think there is enough evidence to go forward on a criminal charge, there may be enough evidence to sue him. I don't know if you remember this, but after OJ Simpson was acquitted of murder, the families of his two victims were successful in suing him in a wrongful death case in civil court
. I realize that you want to see him in jail, but if the prosecutors won't play ball, this is a way to make him suffer at least financially.