Steps to add the custom button and perform Mass Delete of selected record.

Step 1:   Create a lightning component and add following code.

<aura:component implements="force:appHostable,flexipage:availableForAllPageTypes,flexipage:availableForRecordHome,force:hasRecordId,forceCommunity:availableForAllPageTypes" access="global" controller="accListClass" >
    <aura:attribute name="account" type="List"></aura:attribute>
    <aura:handler name="init" value="{!this}" action="{!c.accountList}"></aura:handler>
    <div class="slds-grid">
        <div class="slds-col slds-p-horizontal_small slds-size_1-of-2 slds-medium-size_5-of-6 slds-large-size_8-of-12"></div>
        <div class="slds-col slds-size_1-of-8 allBtn"><lightning:button label="Delete Selected"
                                              iconName="utility:delete"
                                              iconPosition="left"
                                              variant="destructive"
                                              onclick="{!c.deleteSlctd}">
                            </lightning:button></div>
    </div>
    <table class="slds-table slds-table--bordered slds-table--striped slds-table--cell-buffer slds-table--fixed-layout">
        <thead>
        	<tr class="slds-text-heading--label">
                <th scope="row" style="width: 50px;" class="slds-text-align--right"><ui:inputCheckbox aura:id="cbox" class="cBox" change="{!c.selectAll}"/></th>
                <th scope="row"><div class="slds-truncate" title="ID">ID</div></th>
                <th scope="row"><div class="slds-truncate" titile="NAME">Name</div></th>
                <th scope="row"><div class="slds-truncate" title="INDUSTRY">Industry</div></th>
                <th scope="row"><div class="slds-truncate" title="PHONE">Phone</div></th>
            </tr>
        </thead>
        <tbody>
            <aura:iteration items="{!v.account}" var="a">
            	<tr>
                    <td><ui:inputCheckbox aura:id="cboxRow" text="{!a.Id}" class="cBox" change="{!c.changeSelectAll}"/></td>
                	<th><div class="slds-truncate">{!a.Id}</div></th>
                    <td><div class="slds-truncate">{!a.Name}</div></td>
                    <td><div class="slds-truncate">{!a.Industry}</div></td>
                    <td><div class="slds-truncate">{!a.Phone}</div></td>
                </tr>
            </aura:iteration>
        </tbody>
    </table>
</aura:component>

Step 2: Create the client side controller of above mentioned Component and add the following code.

({
    accountList : function(component, event, helper) {
        helper.getAccountList(component);
    },
    selectAll: function(component,event, helper){
        var slctCheck = event.getSource().get("v.value");
        var getCheckAllId = component.find("cboxRow");
        
        if (slctCheck == true) {
            for (var i = 0; i < getCheckAllId.length; i++) {
                component.find("cboxRow")[i].set("v.value", true);             
            }
        } else {
            for (var i = 0; i < getCheckAllId.length; i++) {
                component.find("cboxRow")[i].set("v.value", false);
            }
        }
    },
    changeSelectAll:function(component,event, helper){
        var slctCheckRow = event.getSource().get("v.value");
        var getCheckAllId = component.find("cbox");
        if(slctCheckRow == false) {
            component.find("cbox").set("v.value", false);
        }
    },
    deleteSlctd : function(component,event,helper) {
        var getCheckAllId = component.find("cboxRow");
        var selctedRec = [];
        for (var i = 0; i < getCheckAllId.length; i++) {
            
            if(getCheckAllId[i].get("v.value") == true )
            {
                selctedRec.push(getCheckAllId[i].get("v.text")); 
            }
        }
        helper.deleteSelected(component,event,selctedRec);
    }
})

Step 3: Now create the helper controller of above mentioned Component and add the following code.

({
    getAccountList : function(component) {
        var action = component.get("c.getAccounts");
        var self = this;
        action.setCallback(this, function(actionResult){
            component.set("v.account",actionResult.getReturnValue());
        });
        $A.enqueueAction(action);
    },
    deleteSelected : function(component,event,selctedRec){
        var action = component.get("c.delSlctRec");
        action.setParams({
            "slctRec": selctedRec
        });
        action.setCallback(this, function(response){
            var state =  response.getState();
            if(state == "SUCCESS")
            {
                component.set("v.account",response.getReturnValue());
                console.log("Successfully Deleted..");
            }else if (state=="ERROR") {
                console.log(action.getError()[0].message);
        });
        $A.enqueueAction(action);
    }
})

Step 4: Last step create the apex class name accListClass and add the following code.

public class accListClass{
    @AuraEnabled 
    public List<Account> accList = new List<Account>();
    
    @AuraEnabled 
    public List<Account> delRec = new List<Account>();
    
    @AuraEnabled 
    public static List<Account> getAccounts(String query)
    {
        accListJsClass alc = new accListJsClass();
        
        alc.accList = Database.query('SELECT Id,Name,Phone,Industry from Account');
                
        return alc.accList;
    }
   
    @AuraEnabled 
    public static List<Account> delSlctRec(List<String> slctRec)
    {
        accListJsClass alc = new accListJsClass();
        alc.delRec = [SELECT Id FROM Account WHERE Id IN: slctRec ];
       try{
           delete alc.delRec;
           } catch(Exception ex)
          {
             throw new AuraHandledException(ex.getMessage());
          }
        alc.accList = Database.query('SELECT Id,Name,Phone,Industry from Account');
                
        return alc.accList;
       
    }
}