(Assuming you send correct _id from client) you have to reconstruct the _id as BSON object.
Do not know what db-driver you use, but in mongoskin
it goes like that:
var mongo = require('mongoskin');
var BSON = mongo.BSONPure;
...
var proper_id = BSON.ObjectID(req.params.todo_id)
In mangoose
try following:
var todo = Todo.find({_id : req.params.todo_id});
todo.remove(callback(err, todo)); // callback is optional