Possibly. You can always break a lease, the question is whether the landlord will go after you for any unpaid rent, the remainder of the lease term, and any damages to the residence. The landlord has a duty to insure your quiet enjoyment of the premises. The landlord will argue that they are not doing anything wrong and can not control the third party's actions.
You need to look out for your own safety first and foremost. I would suggest that you notify the landlord of the reasons for reasons for terminating the lease - their refusal to get Night Watch - and simply move out. If they sue you in small claims court, it is their burden of proof and you have a decent affirmative defense to terminating the lease.
It's not reasonable to expect the landlord to pay for security guards or night watchmen when your rent was set at an amount which did not contemplate these expenses. You can hire security at your expense, but it's not reasonable to expect the landlord to fit that into his or her budget.
Under Colorado law, victims of domestic violence or abuse that seek to break their lease due to fear of imminent danger for themselves or their children may do so if they provide their landlord with evidence of the domestic violence or abuse in the form of a police report written within the last 60 days or a valid protection order AND if they pay their landlord one month's rent within 90 days after vacating the premises.
You may have some luck sitting down with your landlord and explaining this statute to him and suggesting that this statute gives you the right to vacate and that you will pay the penalty of one month's rent if he will agree to release you from the lease under these circumstances. He may agree, and if he doesn't, you can still try to argue this statutory provision as a defense to any action brought against you for breaking the lease.
You don't know the identity of your stalker and if it is not someone you have ever been in a relationship with, it may not actually fit the definition of domestic violence. However, you don't know that this is not someone you have been involved with in the past. It could fall within the definition of domestic violence. It is worth making the argument and a judge may be sympathetic to your situation.
You can reach Harkess & Salter LLC at (303) 531-5380 or info@Harkess-Salter.com. Stephen Harkess is an attorney licensed in the state and federal courts of Colorado. This answer is for general information only and does not create an attorney client relationship between Stephen Harkess or Harkess & Salter LLC and any person. You should schedule a consultation with an attorney to discuss the specifics of your legal issues.