Unsubscribing Multiple Subscription from Different Functions

It would be cleaner to use a seperate subscription - and if you use the same field you would never (manually) unsubscribe from the first subscription. Also if you don't want to clutter your component with lots of fields, that simply hold subscription references I would recommend to use a pattern, that involves using just one Subject, that is triggered at ngOnDestroy and before each subscribe you would use takeUntil. So your code could look like this:

private ngUnsubscribe = new Subject();

ngOnInit() {
  this.heroService.getHeroes()
                  .takeUntil(this.ngUnsubscribe)
                  .subscribe(
                     heroes => this.heroes = heroes,
                     error =>  this.errorMessage = <any>error);
}

onSelectVillain(event) {
  this.villainService.getVillains()
                     .takeUntil(this.ngUnsubscribe)
                     .subscribe(
                     .....
}

ngOnDestroy(){
    this.ngUnsubscribe.next();
    this.ngUnsubscribe.complete();
}

See this for further reference.

Note that subscriptions that are "finite", so where a complete state is called, don't neccessarily need to get unsubscribed manually. This is probably a good reference point for that.